fix: ensure last-invoked option wins for shared dest names

src/click/core.py

@@ -1223,6 +1223,19 @@ def parse_args(self, ctx: Context, args: list[str]) -> list[str]:
         parser = self.make_parser(ctx)
         opts, args, param_order = parser.parse_args(args=args)
 
+        # When multiple parameters target the same name (dest), the parser
+        # stores only the last-written value. To ensure the last-invoked
+        # parameter on the command line takes priority during processing,
+        # keep only the last invocation per name in param_order. Earlier
+        # invocations will process as uninvoked (falling through to
+        # defaults), and the existing guard in handle_parse_result
+        # prevents them from overwriting the winning value.
+        # Refs: https://github.com/pallets/click/issues/2786
+        last_for_name: dict[str | None, Parameter] = {}
+        for param in param_order:
+            last_for_name[param.name] = param
+        param_order = [p for p in param_order if last_for_name.get(p.name) is p]
+
         for param in iter_params_for_processing(param_order, self.get_params(ctx)):
             _, args = param.handle_parse_result(ctx, opts, args)
 

tests/test_basic.py

@@ -328,6 +328,45 @@ def cli(case):
     assert result.output == repr(expected)
 
 
+@pytest.mark.parametrize(
+    ("args", "expected"),
+    (
+        # --fetch alone: callback transforms sentinel to fetched value.
+        (["--fetch"], "foo"),
+        # --custom alone: value passed through as-is.
+        (["--custom", "bar"], "bar"),
+        # --custom first, --fetch last: last option wins, callback runs.
+        (["--custom", "bar", "--fetch"], "foo"),
+        # --fetch first, --custom last: last option wins.
+        (["--fetch", "--custom", "bar"], "bar"),
+        # Neither specified: default is None.
+        ([], None),
+    ),
+)
+def test_dual_option_callback_last_wins(runner, args, expected):
+    """When two options share a dest and one has a callback, the last
+    option specified on the command line should win.
+
+    Regression test for https://github.com/pallets/click/issues/2786
+    """
+    SENTINEL = "$_fetch"
+
+    def callback(ctx, param, value):
+        if value is SENTINEL:
+            return "foo"
+        return value
+
+    @click.command()
+    @click.option("--custom", "custom")
+    @click.option("--fetch", "custom", flag_value=SENTINEL, callback=callback)
+    def cli(custom):
+        click.echo(repr(custom), nl=False)
+
+    result = runner.invoke(cli, args)
+    assert result.exit_code == 0, result.output
+    assert result.output == repr(expected)
+
+
 def test_file_option(runner):
     @click.command()
     @click.option("--file", type=click.File("w"))