diff --git a/src/fparser/two/symbol_table.py b/src/fparser/two/symbol_table.py
index a0d6876..c7de860 100644
--- a/src/fparser/two/symbol_table.py
+++ b/src/fparser/two/symbol_table.py
@@ -1,7 +1,7 @@
 # -----------------------------------------------------------------------------
 # BSD 3-Clause License
 #
-# Copyright (c) 2021, Science and Technology Facilities Council.
+# Copyright (c) 2021-2022, Science and Technology Facilities Council.
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
@@ -59,6 +59,9 @@ class SymbolTables():
         self._scoping_unit_classes = []
         # The symbol table of the current scope
         self._current_scope = None
+        # Whether or not we enable consistency checks in the symbol tables
+        # that are created.
+        self._enable_checks = False
 
     def __str__(self):
         result = ("SymbolTables: {0} tables\n"
@@ -66,6 +69,16 @@ class SymbolTables():
                       len(self._symbol_tables)))
         return result + "\n".join(sorted(self._symbol_tables.keys()))
 
+    def enable_checks(self, value):
+        '''
+        Sets whether or not to enable consistency checks in every symbol
+        table that is created during a parse.
+
+        :param bool value: whether or not checks are enabled.
+
+        '''
+        self._enable_checks = value
+
     def clear(self):
         '''
         Deletes any stored SymbolTables but retains the stored list of
@@ -93,7 +106,7 @@ class SymbolTables():
             raise SymbolTableError(
                 "The table of top-level (un-nested) symbol tables already "
                 "contains an entry for '{0}'".format(lower_name))
-        table = SymbolTable(lower_name)
+        table = SymbolTable(lower_name, checking_enabled=self._enable_checks)
         self._symbol_tables[lower_name] = table
         return table
 
@@ -135,7 +148,7 @@ class SymbolTables():
         if not isinstance(value, list):
             raise TypeError("Supplied value must be a list but got '{0}'".
                             format(type(value).__name__))
-        if not all([isinstance(item, type) for item in value]):
+        if not all(isinstance(item, type) for item in value):
             raise TypeError("Supplied list must contain only classes but "
                             "got: {0}".format(value))
         self._scoping_unit_classes = value
@@ -173,7 +186,8 @@ class SymbolTables():
         else:
             # We are already inside a scoping region so create a new table
             # and setup its parent/child connections.
-            table = SymbolTable(lname, parent=self._current_scope)
+            table = SymbolTable(lname, parent=self._current_scope,
+                                checking_enabled=self._enable_checks)
             self._current_scope.add_child(table)
 
         # Finally, make this new table the current scope
@@ -242,10 +256,17 @@ class SymbolTable():
     '''
     Class implementing a single symbol table.
 
+    Since this functionality is not yet fully mature, checks that new symbols
+    don't clash with existing symbols are disabled by default.
+    Once #201 is complete it is planned to switch this so that the checks
+    are instead enabled by default.
+
     :param str name: the name of this scope. Will be the name of the \
                      associated module or routine.
     :param parent: the symbol table within which this one is nested (if any).
     :type parent: :py:class:`fparser.two.symbol_table.SymbolTable.Symbol`
+    :param bool checking_enabled: whether or not validity checks are \
+        performed for symbols added to the table.
 
     '''
     # TODO #201 add support for other symbol properties (kind, shape
@@ -253,7 +274,7 @@ class SymbolTable():
     # type checking for the various properties.
     Symbol = namedtuple("Symbol", "name primitive_type")
 
-    def __init__(self, name, parent=None):
+    def __init__(self, name, parent=None, checking_enabled=False):
         self._name = name.lower()
         # Symbols defined in this scope that represent data.
         self._data_symbols = {}
@@ -263,6 +284,8 @@ class SymbolTable():
         # value (if any) is set via setter method.
         self._parent = None
         self.parent = parent
+        # Whether or not to perform validity checks when symbols are added.
+        self._checking_enabled = checking_enabled
         # Symbol tables nested within this one.
         self._children = []
 
@@ -301,18 +324,23 @@ class SymbolTable():
             raise TypeError(
                 "The primitive type of the symbol must be specified as a str "
                 "but got '{0}'".format(type(primitive_type).__name__))
+
         lname = name.lower()
-        if lname in self._data_symbols:
-            raise SymbolTableError("Symbol table already contains a symbol for"
-                                   " a variable with name '{0}'".format(name))
-        if lname in self._modules:
-            raise SymbolTableError("Symbol table already contains a use of a "
-                                   "module with name '{0}'".format(name))
-        for mod_name in self._modules:
-            if self._modules[mod_name] and lname in self._modules[mod_name]:
+
+        if self._checking_enabled:
+            if lname in self._data_symbols:
+                raise SymbolTableError(
+                    f"Symbol table already contains a symbol for"
+                    f" a variable with name '{name}'")
+            if lname in self._modules:
                 raise SymbolTableError(
-                    "Symbol table already contains a use of a symbol named "
-                    "'{0}' from module '{1}'".format(name, mod_name))
+                    f"Symbol table already contains a use of a "
+                    f"module with name '{name}'")
+            for mod_name, var_list in self._modules.items():
+                if var_list and lname in var_list:
+                    raise SymbolTableError(
+                        f"Symbol table already contains a use of a symbol "
+                        f"named '{name}' from module '{mod_name}'")
 
         self._data_symbols[lname] = SymbolTable.Symbol(lname,
                                                        primitive_type.lower())
@@ -342,7 +370,7 @@ class SymbolTable():
             raise TypeError("If present, the only_list must be a list but got "
                             "'{0}'".format(type(only_list).__name__))
         if only_list and not all(
-                [isinstance(item, str) for item in only_list]):
+                isinstance(item, str) for item in only_list):
             raise TypeError("If present, the only_list must be a list of str "
                             "but got: {0}".format(
                                 [type(item).__name__ for item in only_list]))
@@ -481,6 +509,7 @@ class SymbolTable():
             current = current.parent
         return current
 
+
 #: The single, global container for all symbol tables constructed while
 #: parsing.
 SYMBOL_TABLES = SymbolTables()

diff --git a/src/fparser/two/tests/test_symbol_table.py b/src/fparser/two/tests/test_symbol_table.py
index 879c6c0..06ef1d2 100644
--- a/src/fparser/two/tests/test_symbol_table.py
+++ b/src/fparser/two/tests/test_symbol_table.py
@@ -1,5 +1,5 @@
 # -----------------------------------------------------------------------------
-# Copyright (c) 2021 Science and Technology Facilities Council
+# Copyright (c) 2021-2022 Science and Technology Facilities Council.
 # All rights reserved.
 #
 # Modifications made as part of the fparser project are distributed
@@ -49,6 +49,8 @@ def test_basic_table():
     assert table.name == "basic"
     assert table.parent is None
     assert table.children == []
+    # Consistency checking is disabled by default
+    assert table._checking_enabled is False
     with pytest.raises(KeyError) as err:
         table.lookup("missing")
     assert "Failed to find symbol named 'missing'" in str(err.value)
@@ -57,11 +59,15 @@ def test_basic_table():
     sym = table.lookup("var")
     assert sym.name == "var"
     assert table.lookup("VAR") is sym
+    # Check that we can enable consistency checking
+    table2 = SymbolTable("table2", checking_enabled=True)
+    assert table2._checking_enabled is True
 
 
 def test_add_data_symbol():
-    ''' Test that the add_data_symbol() method behaves as expected. '''
-    table = SymbolTable("basic")
+    ''' Test that the add_data_symbol() method behaves as expected when
+    validation is enabled. '''
+    table = SymbolTable("basic", checking_enabled=True)
     table.add_data_symbol("var", "integer")
     sym = table.lookup("var")
     assert sym.primitive_type == "integer"
@@ -90,6 +96,21 @@ def test_add_data_symbol():
             "module 'mod1'" in str(err.value))
 
 
+def test_add_data_symbols_no_checks():
+    ''' Check that we can disable the checks in the
+    add_data_symbol() method. '''
+    table = SymbolTable("basic", checking_enabled=False)
+    table.add_data_symbol("var", "integer")
+    table.add_data_symbol("var", "real")
+    sym = table.lookup("var")
+    assert sym.primitive_type == "real"
+    table.add_use_symbols("mod1", ["var3"])
+    table.add_data_symbol("mod1", "real")
+    table.add_use_symbols("mod2", ["var3"])
+    table.add_data_symbol("var3", "real")
+    assert table.lookup("var3").primitive_type == "real"
+
+
 def test_add_use_symbols():
     ''' Test that the add_use_symbols() method behaves as expected. '''
     table = SymbolTable("basic")
diff --git a/src/fparser/two/tests/test_symbol_tables.py b/src/fparser/two/tests/test_symbol_tables.py
index c209ab4..e18f079 100644
--- a/src/fparser/two/tests/test_symbol_tables.py
+++ b/src/fparser/two/tests/test_symbol_tables.py
@@ -1,5 +1,5 @@
 # -----------------------------------------------------------------------------
-# Copyright (c) 2021 Science and Technology Facilities Council
+# Copyright (c) 2021-2022 Science and Technology Facilities Council.
 # All rights reserved.
 #
 # Modifications made as part of the fparser project are distributed
@@ -48,6 +48,7 @@ def test_construction_addition_removal():
     tables = SymbolTables()
     assert tables._current_scope is None
     assert tables._symbol_tables == {}
+    assert tables._enable_checks is False
     with pytest.raises(KeyError) as err:
         tables.lookup("missing")
     assert "missing" in str(err.value)
@@ -62,10 +63,16 @@ def test_construction_addition_removal():
             "an entry for 'table1'" in str(err.value))
     # Add a second table and then remove it
     table2 = tables.add("taBLe2")
+    # Check that validation checks are disabled by default
+    assert table2._checking_enabled is False
     assert tables.lookup("table2") is table2
     tables.remove("table2")
     with pytest.raises(KeyError) as err:
         tables.lookup("table2")
+    # Turn on validation checking
+    tables.enable_checks(True)
+    table3 = tables.add("table3")
+    assert table3._checking_enabled is True
     # Clear the stored symbol tables
     tables.clear()
     assert tables._current_scope is None

Allow symbol checks to be disabled
The new symbol table functionality is somewhat immature (it cannot cope with use association) and only works if the supplied source is valid Fortran, even if any pre-processor directives are ignored. These limitations mean that it can be tripped up in certain circumstances. While support for use association is planned (#201 and #349), there's no plan to handle CPP directives. Therefore we will allow the checking performed by the symbol table to be disabled at run time.

I'm not sure whether this will be best done by completely disabling the symbol table functionality or by only disabling the checks that a symbol does not already exist.
src/fparser/two/tests/test_symbol_table.py::test_basic_table src/fparser/two/tests/test_symbol_table.py::test_add_data_symbol src/fparser/two/tests/test_symbol_table.py::test_add_data_symbols_no_checks src/fparser/two/tests/test_symbol_tables.py::test_construction_addition_removal
src/fparser/two/tests/test_symbol_table.py::test_add_use_symbols src/fparser/two/tests/test_symbol_table.py::test_add_use_symbols_errors src/fparser/two/tests/test_symbol_table.py::test_str_method src/fparser/two/tests/test_symbol_table.py::test_del_child src/fparser/two/tests/test_symbol_table.py::test_parent_child src/fparser/two/tests/test_symbol_table.py::test_root_property src/fparser/two/tests/test_symbol_table.py::test_module_use src/fparser/two/tests/test_symbol_table.py::test_module_use_with_only src/fparser/two/tests/test_symbol_table.py::test_module_definition src/fparser/two/tests/test_symbol_table.py::test_routine_in_module src/fparser/two/tests/test_symbol_table.py::test_routine_in_prog src/fparser/two/tests/test_symbol_tables.py::test_scoping_unit_classes_setter src/fparser/two/tests/test_symbol_tables.py::test_str_method src/fparser/two/tests/test_symbol_tables.py::test_nested_scoping src/fparser/two/tests/test_symbol_tables.py::test_nested_removal