prevent duplicate plugin discovery on misconfigured pythons

for example, `venv`-virtualenvs on fedora have both `lib` and `lib64` on
`sys.path` despite them being the same.  this causes
`importlib.metadata.distributions` to double-discover.

```console
$ docker run --rm -t fedora:latest bash -c 'dnf install -qq -y python3 >& /dev/null && python3 -m venv venv && venv/bin/pip -qq install cfgv && venv/bin/python - <<< "from importlib.metadata import distributions; print(len([d for d in distributions() if d.name == '"'"'cfgv'"'"']))"'
2
```

src/flake8/plugins/finder.py

@@ -179,6 +179,8 @@ def _flake8_plugins(
 
 
 def _find_importlib_plugins() -> Generator[Plugin, None, None]:
+    # some misconfigured pythons (RHEL) have things on `sys.path` twice
+    seen = set()
     for dist in importlib_metadata.distributions():
         # assigned to prevent continual reparsing
         eps = dist.entry_points
@@ -190,6 +192,11 @@ def _find_importlib_plugins() -> Generator[Plugin, None, None]:
         # assigned to prevent continual reparsing
         meta = dist.metadata
 
+        if meta["name"] in seen:
+            continue
+        else:
+            seen.add(meta["name"])
+
         if meta["name"] in BANNED_PLUGINS:
             LOG.warning(
                 "%s plugin is obsolete in flake8>=%s",

tests/unit/plugins/finder_test.py

@@ -361,6 +361,23 @@ unrelated = unrelated:main
     ]
 
 
+def test_duplicate_dists(flake8_dist):
+    # some poorly packaged pythons put lib and lib64 on sys.path resulting in
+    # duplicates from `importlib.metadata.distributions`
+    with mock.patch.object(
+        importlib_metadata,
+        "distributions",
+        return_value=[
+            flake8_dist,
+            flake8_dist,
+        ],
+    ):
+        ret = list(finder._find_importlib_plugins())
+
+    # we should not have duplicates
+    assert len(ret) == len(set(ret))
+
+
 def test_find_local_plugins_nothing():
     cfg = configparser.RawConfigParser()
     assert set(finder._find_local_plugins(cfg)) == set()