Support passing the same key multiple times in --config-settings

Signed-off-by: Filipe Laíns <lains@riseup.net>

src/pip/_internal/cli/cmdoptions.py

@@ -811,7 +811,13 @@ def _handle_config_settings(
     if dest is None:
         dest = {}
         setattr(parser.values, option.dest, dest)
-    dest[key] = val
+    if key in dest:
+        if isinstance(dest[key], list):
+            dest[key].append(val)
+        else:
+            dest[key] = [dest[key], val]
+    else:
+        dest[key] = val
 
 
 config_settings: Callable[..., Option] = partial(

tests/unit/test_pyproject_config.py

@@ -36,9 +36,16 @@ def test_set_config_empty_value() -> None:
     assert options.config_settings == {"x": ""}
 
 
-def test_replace_config_value() -> None:
+@pytest.mark.parametrize(
+    ("passed", "expected"),
+    [
+        (["x=hello", "x=world"], {"x": ["hello", "world"]}),
+        (["x=hello", "x=world", "x=other"], {"x": ["hello", "world", "other"]}),
+    ],
+)
+def test_multiple_config_values(passed, expected) -> None:
     i = create_command("install")
     options, _ = i.parse_args(
-        ["xxx", "--config-settings", "x=hello", "--config-settings", "x=world"]
+        ["xxx", *(f"--config-settings={option}" for option in passed)]
     )
-    assert options.config_settings == {"x": "world"}
+    assert options.config_settings == expected