diff --git a/bibtexparser/middlewares/__init__.py b/bibtexparser/middlewares/__init__.py
index 89b0af1..3fd1a36 100644
--- a/bibtexparser/middlewares/__init__.py
+++ b/bibtexparser/middlewares/__init__.py
@@ -1,5 +1,6 @@
 from bibtexparser.middlewares.enclosing import AddEnclosingMiddleware
 from bibtexparser.middlewares.enclosing import RemoveEnclosingMiddleware
+from bibtexparser.middlewares.fieldkeys import NormalizeFieldKeys
 from bibtexparser.middlewares.interpolate import ResolveStringReferencesMiddleware
 from bibtexparser.middlewares.latex_encoding import LatexDecodingMiddleware
 from bibtexparser.middlewares.latex_encoding import LatexEncodingMiddleware
diff --git a/bibtexparser/middlewares/fieldkeys.py b/bibtexparser/middlewares/fieldkeys.py
new file mode 100644
index 0000000..edb5a7e
--- /dev/null
+++ b/bibtexparser/middlewares/fieldkeys.py
@@ -0,0 +1,52 @@
+import logging
+from typing import Dict
+from typing import List
+from typing import Set
+
+from bibtexparser.library import Library
+from bibtexparser.model import Entry
+from bibtexparser.model import Field
+
+from .middleware import BlockMiddleware
+
+
+class NormalizeFieldKeys(BlockMiddleware):
+    """Normalize field keys to lowercase.
+
+    In case of conflicts (e.g. both 'author' and 'Author' exist in the same entry),
+    a warning is emitted, and the last value wins.
+
+    Some other middlewares, such as `SeparateCoAuthors`, assume lowercase key names.
+    """
+
+    def __init__(self, allow_inplace_modification: bool = True):
+        super().__init__(
+            allow_inplace_modification=allow_inplace_modification,
+            allow_parallel_execution=True,
+        )
+
+    # docstr-coverage: inherited
+    def transform_entry(self, entry: Entry, library: "Library") -> Entry:
+        seen_normalized_keys: Set[str] = set()
+        new_fields_dict: Dict[str, Field] = {}
+        for field in entry.fields:
+            normalized_key: str = field.key.lower()
+            # if the normalized key is already present, apply "last one wins"
+            # otherwise preserve insertion order
+            # if a key is overwritten, emit a detailed warning
+            # if performance is a concern, we could emit a warning with only {entry.key}
+            # to remove "seen_normalized_keys" and this if statement
+            if normalized_key in seen_normalized_keys:
+                logging.warning(
+                    f"NormalizeFieldKeys: in entry '{entry.key}': "
+                    + f"duplicate normalized key '{normalized_key}' "
+                    + f"(original '{field.key}'); overriding previous value"
+                )
+            seen_normalized_keys.add(normalized_key)
+            field.key = normalized_key
+            new_fields_dict[normalized_key] = field
+
+        new_fields: List[Field] = list(new_fields_dict.values())
+        entry.fields = new_fields
+
+        return entry

diff --git a/tests/middleware_tests/test_fieldkeys.py b/tests/middleware_tests/test_fieldkeys.py
new file mode 100644
index 0000000..acff042
--- /dev/null
+++ b/tests/middleware_tests/test_fieldkeys.py
@@ -0,0 +1,68 @@
+import re
+
+from bibtexparser import Library
+from bibtexparser.middlewares.fieldkeys import NormalizeFieldKeys
+from bibtexparser.model import Entry
+from bibtexparser.model import Field
+
+entries = {
+    "article": {
+        "author": '"Smith, J."',
+        "title": '"A Test Article"',
+        "journal": '"J. of Testing"',
+        "month": '"jan"',
+        "year": '"2022"',
+    },
+    "book": {
+        "author": '"Doe, J."',
+        "title": '"A Test Book"',
+        "publisher": '"Test Pub."',
+        "year": '"2021"',
+        "month": "apr",
+    },
+    "inproceedings": {
+        "author": '"Jones, R."',
+        "title": '"A Test Conf. Paper"',
+        "booktitle": '"Proc. of the Intl. Test Conf."',
+        "year": '"2023"',
+        "month": "8",
+    },
+}
+
+ref = Library()
+for i, (entry_type, fields) in enumerate(entries.items()):
+    f = [Field(key=k, value=v) for k, v in fields.items()]
+    ref.add(Entry(entry_type=entry_type, key=f"entry{i}", fields=f))
+
+
+def test_normalize_fieldkeys():
+    """
+    Check library with lowercase field keys.
+    """
+
+    lib = Library()
+    for i, (entry_type, fields) in enumerate(entries.items()):
+        f = [Field(key=k, value=v) for k, v in fields.items()]
+        lib.add(Entry(entry_type=entry_type, key=f"entry{i}", fields=f))
+
+    lib = NormalizeFieldKeys().transform(lib)
+
+    for key in lib.entries_dict:
+        assert lib.entries_dict[key] == ref.entries_dict[key]
+
+
+def test_normalize_fieldkeys_force_last(caplog):
+    """
+    Check library with uppercase field keys and duplicate normalized keys.
+    """
+    lib = Library()
+    for i, (entry_type, fields) in enumerate(entries.items()):
+        f = [Field(key=k.lower(), value="dummyvalue") for k in fields]
+        f += [Field(key=k.upper(), value=v) for k, v in fields.items()]
+        lib.add(Entry(entry_type=entry_type, key=f"entry{i}", fields=f))
+
+    lib = NormalizeFieldKeys().transform(lib)
+    assert re.match(r"(WARNING\s*)(\w*\:\w*\.py\:[0-9]*\s*)(NormalizeFieldKeys)(.*)", caplog.text)
+
+    for key in lib.entries_dict:
+        assert lib.entries_dict[key] == ref.entries_dict[key]

Normalize field keys (to lowercase)
**Describe the bug**
I have several .bib files that contain (mixed) field keys that are either in lowercase or start with a capital letter, such as "Author" and "Title". No other tooling complains about this.

SeparateCoAuthors does not work and I cannot uniformy access the fields using e.g. `entry['title']`

A normalization to lowercase of the field keys was conducted in v1.

Maybe this can be fixed using a middleware? I would be really grateful!

**Reproducing**

Version: e3757c13abf2784bda612464843ab30256317e6c

Code:
```python

#!/usr/bin/python

import bibtexparser
import bibtexparser.middlewares as m

layers = [
	m.LatexDecodingMiddleware(),
	m.MonthIntMiddleware(True), # Months should be represented as int (0-12)
	m.SeparateCoAuthors(True), # Co-authors should be separated as list of strings
	m.SplitNameParts(True), # Individual Names should be split into first, von, last, jr parts
	m.MergeNameParts("last", True) # Individual Names should be merged oto Last, First...
]

bib_database = bibtexparser.parse_file('data/Survey.bib', append_middleware=layers)
for entry in bib_database.entries:
	print(entry['title']);
```

Bibtex:
```
@InCollection{Name2006,
  Title                    = {A Title},
  Author                   = {Name, First and Name, Second},
  Booktitle                = {ITS},
  Publisher                = {Some publisher},
  Year                     = {2006},
  Pages                    = {61--70}
}
```

**Remaining Questions (Optional)**
Please tick all that apply:

- [ ] I would be willing to contribute a PR to fix this issue.
- [ ] This issue is a blocker, I'd be grateful for an early fix.

tests/middleware_tests/test_fieldkeys.py::test_normalize_fieldkeys tests/middleware_tests/test_fieldkeys.py::test_normalize_fieldkeys_force_last