diff --git a/dvc/repo/__init__.py b/dvc/repo/__init__.py
--- a/dvc/repo/__init__.py
+++ b/dvc/repo/__init__.py
@@ -24,6 +24,7 @@ def locked(f):
     @wraps(f)
     def wrapper(repo, *args, **kwargs):
         with repo.lock, repo.state:
+            repo._reset()
             ret = f(repo, *args, **kwargs)
             # Our graph cache is no longer valid after we release the repo.lock
             repo._reset()
diff --git a/dvc/repo/add.py b/dvc/repo/add.py
--- a/dvc/repo/add.py
+++ b/dvc/repo/add.py
@@ -126,6 +126,7 @@ def _create_stages(repo, targets, fname, pbar=None):
         stage = Stage.create(
             repo, outs=[out], accompany_outs=True, fname=fname
         )
+        repo._reset()
 
         if not stage:
             if pbar is not None:

Thanks for reporting this, @jaredsampson !

Including the stack trace:

```
ERROR: file/directory 'data' is specified as an output in more than one stage: data.dvc
    data.dvc
------------------------------------------------------------
Traceback (most recent call last):
  File "/dvc/command/add.py", line 25, in run
    fname=self.args.file,
  File "/dvc/repo/__init__.py", line 36, in wrapper
    ret = f(repo, *args, **kwargs)
  File "/dvc/repo/scm_context.py", line 4, in run
    result = method(repo, *args, **kw)
  File "/dvc/repo/add.py", line 50, in add
    repo.check_modified_graph(stages)
  File "/dvc/repo/__init__.py", line 177, in check_modified_graph
    self._collect_graph(self.stages + new_stages)
  File "/dvc/repo/__init__.py", line 327, in _collect_graph
    raise OutputDuplicationError(str(out), stages)
dvc.exceptions.OutputDuplicationError: file/directory 'data' is specified as an output in more than one stage: data.dvc
    data.dvc
------------------------------------------------------------
```
I get the same behavior with a flat file foo.txt instead of `data/foo` as well:

```
dvc init --no-scm
mkdir data
echo "foo" > foo.txt
dvc add foo.txt
echo "bar" > bar.txt
dvc unprotect foo.txt
echo "change" > foo.txt
dvc add foo.txt bar.txt
```

yields 

```
WARNING: Output 'foo.txt' of 'foo.txt.dvc' changed because it is 'modified'
100%|██████████|Add                                       2.00/2.00 [00:01<00:00,  1.51file/s]
ERROR: file/directory 'foo.txt' is specified as an output in more than one stage: foo.txt.dvc
    foo.txt.dvc
```

However, reversing the order of the files in the last command, everything works fine:

```
$ dvc add foo.txt bar.txt
WARNING: Output 'foo.txt' of 'foo.txt.dvc' changed because it is 'modified'
100% Add                                                  2.00/2.00 [00:01<00:00,  1.47file/s]
```
Thanks for the additional info, @jaredsampson !

The problem seems to be related to the way we collect the stages.
```
#!/bin/bash

set -e
set -x

rm -rf myrepo
mkdir myrepo
cd myrepo

dvc init --no-scm

mkdir data

echo "foo" > foo.txt
dvc add foo.txt

echo "bar" > bar.txt
dvc unprotect foo.txt

echo "change" > foo.txt
dvc add foo.txt bar.txt
```

doesn't reproduce it for me 🙁I suspect that the original issue might be cause by us not deduping the targets in dvc/repo/add.py, but I haven't looked into it closely enough.
@efiop , change the order of the last call and it should work:
```diff
- dvc add foo.txt bar.txt
+ dvc add bar.txt foo.txt
```
If someone is willing to give it a try, here's a test that you can play with:

```diff
diff --git a/tests/func/test_add.py b/tests/func/test_add.py
index 6d51f233..7328c84d 100644
--- a/tests/func/test_add.py
+++ b/tests/func/test_add.py
@@ -24,7 +24,7 @@ from dvc.system import System
 from dvc.utils import file_md5
 from dvc.utils import LARGE_DIR_SIZE
 from dvc.utils import relpath
-from dvc.utils.compat import range
+from dvc.utils.compat import range, pathlib
 from dvc.utils.stage import load_stage_file
 from tests.basic_env import TestDvc
 from tests.utils import get_gitignore_content
@@ -649,3 +649,10 @@ def test_escape_gitignore_entries(git, dvc_repo, repo_dir):
     dvc_repo.add(fname)
 
     assert ignored_fname in get_gitignore_content()
+
+
+def test_adding_several_files_after_one_has_been_modified(dvc_repo):
+    # https://github.com/iterative/dvc/issues/2886
+    dvc_repo.add('foo')
+    pathlib.Path('foo').write_text('change')
+    dvc_repo.add(['bar', 'foo'])
```

Might need revisiting the graph building process :grimacing: 
@iterative/engineering it seems like an important bug. Should we make it p0?
can confirm that problem still exists on master:
```
#!/bin/bash

rm -rf repo
mkdir repo

pushd repo
git init --quiet
dvc init -q

set -ex

mkdir data

echo foo>>data/foo
dvc add -q data

echo bar>>bar
echo change>>data/foo

dvc add bar data
```
Ok, so little investigation helped me to narrow down the issue.
The bug shows up when we are re-`add`-ing existing DVC-tracked output, but only after calling 
`dvc.stages` first. For example:
Latest version i checked with: 0.86.1
```
def test_re_add(tmp_dir, scm, dvc):
    tmp_dir.dvc_gen( {"file": "file content"})

    tmp_dir.gen(
            {"file": "modified content"}
    )
    dvc.stages
    dvc.add(["file"])
```
We will get the same OutputDuplicationError.

1. Why this is happening? 
`repo.stages` is cached property, so repo is storing `file.dvc`. The problem is that when we are re-`add`-ing some file, the chain of calls is `repo.add` -> `repo._create_stages` -> `Stage.create` which detects that `file.dvc` exists and removes it, so it can be created again. 

So now we don't have `file.dvc` because it was removed by `Stage.create`, we have `file.dvc` in memory as "new" stage that we are creating right now, and because `repo`'s stages were not invalidated, we also have `repo.stages`==[`file.dvc`]. That is why we try to check modified graph [here](https://github.com/iterative/dvc/blob/a90d7009f52fa03b45ad5c1d22943cebee387b01/dvc/repo/__init__.py#L173) during `add` we have `self.stages==[`file.dvc`]` and `new_stages==[`file.dvc`]`. While, actually self.stages should not return it (since it was removed by `Stage.create`.

**TLDR**
When re-`add`ing we remove stage file, but we don't invalidate cached `repo.stages` and end up with `repo._collect_graph(['file.dvc', 'file.dvc'])` which has to fail.

Solution: invalidate `repo.stages` when removing the stage file.
diff --git a/tests/func/test_add.py b/tests/func/test_add.py
--- a/tests/func/test_add.py
+++ b/tests/func/test_add.py
@@ -655,3 +655,10 @@ def test_add_from_data_dir(tmp_dir, scm, dvc):
         "tracked output: 'dir'.\n"
         "To include '{out}' in 'dir', run 'dvc commit dir.dvc'"
     ).format(out=os.path.join("dir", "file2"))
+
+
+def test_not_raises_on_re_add(tmp_dir, dvc):
+    tmp_dir.dvc_gen("file", "file content")
+
+    tmp_dir.gen({"file2": "file2 content", "file": "modified file"})
+    dvc.add(["file2", "file"])
diff --git a/tests/unit/repo/test_repo.py b/tests/unit/repo/test_repo.py
--- a/tests/unit/repo/test_repo.py
+++ b/tests/unit/repo/test_repo.py
@@ -2,6 +2,8 @@
 
 import pytest
 
+from dvc.repo import locked
+
 
 def test_is_dvc_internal(dvc):
     assert dvc.is_dvc_internal(os.path.join("path", "to", ".dvc", "file"))
@@ -49,3 +51,18 @@ def test_used_cache(tmp_dir, dvc, path):
             used_cache._items == expected._items
             and used_cache.external == expected.external
         )
+
+
+def test_locked(mocker):
+    repo = mocker.MagicMock()
+    repo.method = locked(repo.method)
+
+    args = {}
+    kwargs = {}
+    repo.method(repo, args, kwargs)
+
+    assert repo.method_calls == [
+        mocker.call._reset(),
+        mocker.call.method(repo, args, kwargs),
+        mocker.call._reset(),
+    ]

Bug: `dvc add` fails with a modified file (or directory) at the end of a list of files
Using DVC 0.71.0 on RHEL6, installed via `conda`, configured for system-wide hard links (not sure if it's relevant):

```
$ dvc --version
0.71.0

$ cat ~/.config/dvc/config
[cache]
protected = true
type = hardlink
```

I have come across what appears to be a bug, where attempting to `dvc add` a directory previously under DVC version control whose contents have changed, results in an error, but only when adding along with a list of files, and doesn't occur if the command is repeated (i.e. after all the other files have been added).  I have reproduced this in several project directories and via the following minimal example (courtesy of @mroutis):

```
dvc init --no-scm
mkdir data
echo "foo" > data/foo
dvc add data
echo "bar" > bar.txt
dvc unprotect data
echo "change" > data/foo
dvc add bar.txt data
```

This results in the following output:

```
WARNING: Output 'data' of 'data.dvc' changed because it is 'modified'
100%|██████████|Add                                       2.00/2.00 [00:02<00:00,  1.08s/file]
ERROR: file/directory 'data' is specified as an output in more than one stage: data.dvc
    data.dvc
```

But if the command is re-run:

```
$ dvc add bar.txt data
Stage is cached, skipping.
100% Add                                                  1.00/1.00 [00:01<00:00,  1.96s/file]
```

So it appears `dvc` is somehow mishandling the list of files.  Of course, the expected behavior is that it will add the directory successfully on the first try.

Thanks for any effort to track down the source of the bug.






tests/func/test_add.py::test_not_raises_on_re_add tests/unit/repo/test_repo.py::test_locked
tests/func/test_add.py::test_add tests/func/test_add.py::test_add_unicode tests/func/test_add.py::test_add_unsupported_file tests/func/test_add.py::test_add_directory tests/func/test_add.py::TestAddDirectoryRecursive::test tests/func/test_add.py::TestAddCmdDirectoryRecursive::test tests/func/test_add.py::TestAddCmdDirectoryRecursive::test_warn_about_large_directories tests/func/test_add.py::TestAddDirectoryWithForwardSlash::test tests/func/test_add.py::test_add_tracked_file tests/func/test_add.py::TestAddDirWithExistingCache::test tests/func/test_add.py::TestAddModifiedDir::test tests/func/test_add.py::test_add_file_in_dir tests/func/test_add.py::TestAddExternalLocalFile::test tests/func/test_add.py::TestAddLocalRemoteFile::test tests/func/test_add.py::TestCmdAdd::test tests/func/test_add.py::TestDoubleAddUnchanged::test_dir tests/func/test_add.py::TestDoubleAddUnchanged::test_file tests/func/test_add.py::test_should_update_state_entry_for_file_after_add tests/func/test_add.py::test_should_update_state_entry_for_directory_after_add tests/func/test_add.py::TestAddCommit::test tests/func/test_add.py::test_should_collect_dir_cache_only_once tests/func/test_add.py::TestShouldAddDataFromExternalSymlink::test tests/func/test_add.py::TestShouldAddDataFromInternalSymlink::test tests/func/test_add.py::TestShouldPlaceStageInDataDirIfRepositoryBelowSymlink::test tests/func/test_add.py::TestShouldThrowProperExceptionOnCorruptedStageFile::test tests/func/test_add.py::TestAddFilename::test tests/func/test_add.py::test_failed_add_cleanup tests/func/test_add.py::test_should_not_track_git_internal_files tests/func/test_add.py::test_readding_dir_should_not_unprotect_all tests/func/test_add.py::test_should_not_checkout_when_adding_cached_copy tests/func/test_add.py::test_should_relink_on_repeated_add[hardlink-copy-<lambda>] tests/func/test_add.py::test_should_relink_on_repeated_add[symlink-copy-<lambda>] tests/func/test_add.py::test_should_relink_on_repeated_add[copy-hardlink-is_hardlink] tests/func/test_add.py::test_should_relink_on_repeated_add[copy-symlink-is_symlink] tests/func/test_add.py::test_escape_gitignore_entries tests/func/test_add.py::test_add_from_data_dir tests/unit/repo/test_repo.py::test_is_dvc_internal tests/unit/repo/test_repo.py::test_find_outs_by_path[dir/subdir/file] tests/unit/repo/test_repo.py::test_find_outs_by_path[dir/subdir] tests/unit/repo/test_repo.py::test_find_outs_by_path[dir] tests/unit/repo/test_repo.py::test_used_cache[dir/subdir/file] tests/unit/repo/test_repo.py::test_used_cache[dir/subdir]