Fix/improve handling of pkg init and test file via args

Ref: https://github.com/pytest-dev/pytest/issues/4344#issuecomment-441095934

changelog/4745.bugfix.rst

@@ -0,0 +1 @@
+Fix/improve collection of args when passing in ``__init__.py`` and a test file.

src/_pytest/main.py

@@ -582,7 +582,7 @@ class Session(nodes.FSCollector):
                 col = self._node_cache[argpath]
             else:
                 collect_root = self._pkg_roots.get(argpath.dirname, self)
-                col = collect_root._collectfile(argpath)
+                col = collect_root._collectfile(argpath, handle_dupes=False)
                 if col:
                     self._node_cache[argpath] = col
             m = self.matchnodes(col, names)

testing/test_collection.py

@@ -1157,3 +1157,32 @@ def test_collectignore_via_conftest(testdir, monkeypatch):
 
     result = testdir.runpytest()
     assert result.ret == EXIT_NOTESTSCOLLECTED
+
+
+def test_collect_pkg_init_and_file_in_args(testdir):
+    subdir = testdir.mkdir("sub")
+    init = subdir.ensure("__init__.py")
+    init.write("def test_init(): pass")
+    p = subdir.ensure("test_file.py")
+    p.write("def test_file(): pass")
+
+    # NOTE: without "-o python_files=*.py" this collects test_file.py twice.
+    # This changed/broke with "Add package scoped fixtures #2283" (2b1410895)
+    # initially (causing a RecursionError).
+    result = testdir.runpytest("-v", str(init), str(p))
+    result.stdout.fnmatch_lines(
+        [
+            "sub/test_file.py::test_file PASSED*",
+            "sub/test_file.py::test_file PASSED*",
+            "*2 passed in*",
+        ]
+    )
+
+    result = testdir.runpytest("-v", "-o", "python_files=*.py", str(init), str(p))
+    result.stdout.fnmatch_lines(
+        [
+            "sub/__init__.py::test_init PASSED*",
+            "sub/test_file.py::test_file PASSED*",
+            "*2 passed in*",
+        ]
+    )