test: try to reproduce --stdout double newlines

src/darker/tests/test_main.py

@@ -942,6 +942,27 @@ def test_stdout_path_resolution(git_repo, capsys):
     assert capsys.readouterr().out == 'print("foo")\n'
 
 
+@pytest.mark.parametrize("newline", ["\n", "\r\n"], ids=["unix", "windows"])
+def test_stdout_newlines(tmp_path, monkeypatch, capsysbinary, newline):
+    """When using ``--stdout``, newlines are not duplicated.
+
+    See: https://github.com/akaihola/darker/issues/604
+
+    """
+    monkeypatch.chdir(tmp_path)
+    code = f"import collections{newline}import sys{newline}".encode()
+    Path("new-file.py").write_bytes(code)
+    with patch("sys.stdin.buffer.read", return_value=code):
+
+        result = darker.__main__.main(
+            ["--stdout", "--isort", "--stdin-filename=new-file.py", "-"],
+        )
+
+    assert result == 0
+    expect = f"import collections{newline}import sys{newline}".encode()
+    assert capsysbinary.readouterr().out == expect
+
+
 def test_long_command_length(git_repo):
     """Large amount of changed files does not break Git invocation even on Windows"""
     # For PR #542 - large character count for changed files