diff --git a/CHANGELOG.md b/CHANGELOG.md
index d642492..403184a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,8 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); ver
 
 ### Fixed
 
+- `decide` rejects repeated `--option` keys before model setup instead of silently replacing an earlier choice
+  description. The usage error names the duplicate key without printing either description.
 - Malformed ticket-router JSONL now names the dataset, physical line and JSON column without printing ticket contents.
 - Laya browser-state compaction preserves the recovery plan for the next decision.
 - Desktop recovery resets its detection window on observed progress without resetting its cumulative budget.
diff --git a/README.md b/README.md
index ce1490d..b465718 100644
--- a/README.md
+++ b/README.md
@@ -120,6 +120,9 @@ uv run s1a run blackjack --model jev --rethink off --episodes 20
 uv run s1a run blackjack --model llm --rethink off --episodes 20     # the chat model in the same agent
 ```
 
+Each `--option KEY=DESCRIPTION` must use a unique, case-sensitive key. Repeated keys are usage errors (exit 2),
+reported before loading a model or checking its credentials; they never replace an earlier description.
+
 `decide` prints one JSON object with `choice`, a probability per option, `confidence` and `ms`; `run` writes a job
 folder with the score. Without a key, `--model cua` answers in process after `uv sync --extra cua`.
 
diff --git a/s1a/cli.py b/s1a/cli.py
index 134d284..1bb4b4b 100644
--- a/s1a/cli.py
+++ b/s1a/cli.py
@@ -44,7 +44,11 @@ def parser() -> argparse.ArgumentParser:
     )
     decide.add_argument("--state", required=True, help="a JSON object, or @path to a file holding one")
     decide.add_argument(
-        "--option", action="append", required=True, metavar="KEY=DESCRIPTION", help="one candidate; repeat per option"
+        "--option",
+        action="append",
+        required=True,
+        metavar="KEY=DESCRIPTION",
+        help="one candidate; repeat per option with a unique key",
     )
     decide.add_argument("--rules", required=True, help="the facts the model applies when it picks")
     decide.add_argument(
@@ -85,6 +89,8 @@ def parse_options(items: list[str]) -> dict[str, str]:
         key, sep, description = item.partition("=")
         if not sep or not key:
             raise ValueError(f"--option needs KEY=DESCRIPTION, got {item!r}")
+        if key in options:
+            raise ValueError(f"--option repeats key {key!r}; use a unique key for each choice")
         options[key] = description
     return options
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 74d1ac7..20ea3dd 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -241,6 +241,21 @@ def test_malformed_input_is_one_line_on_stderr_and_exit_2_before_any_key_check(s
                     if argv[0] == "decide" and argv[2] != "{}":  # the three malformed --state values
                         self.assertTrue(err.startswith("--state"), err)
 
+    def test_duplicate_option_keys_fail_before_building_any_model(self) -> None:
+        for model in cli.DECIDE_MODEL_NAMES:
+            for description in ("keep", "sensitive replacement description"):
+                with self.subTest(model=model, description=description), patch.object(cli, "build_model") as build:
+                    code, out, err = _main([*DECIDE, "--model", model, "--option", f"stand={description}"])
+                    self.assertEqual((code, out), (2, ""))
+                    self.assertEqual(err, "--option repeats key 'stand'; use a unique key for each choice\n")
+                    build.assert_not_called()
+
+    def test_valid_options_preserve_order_case_and_description(self) -> None:
+        options = cli.parse_options(["b=one=two", "A= leading and trailing ", "a=", "路线=支持"])
+        self.assertEqual(
+            list(options.items()), [("b", "one=two"), ("A", " leading and trailing "), ("a", ""), ("路线", "支持")]
+        )
+
     def test_state_from_a_file_and_bad_options(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             path = Path(tmp) / "state.json"