diff --git a/src/host/taint/tests.rs b/src/host/taint/tests.rs
index 10df0c4..eff5f24 100644
--- a/src/host/taint/tests.rs
+++ b/src/host/taint/tests.rs
@@ -241,6 +241,17 @@ fn ownership_carriers_match_the_cross_repo_fixture() {
         .filter(|line| !line.is_empty() && !line.starts_with('#'))
         .map(|line| line.split_once('=').expect("fixture line is key=value"))
         .collect();
+
+    // ⚠️ Refuse a duplicated key rather than resolving it (Codex phase-1
+    // review, finding 3). This side takes the first match and peerspeak's
+    // took the last, so a duplicate in a byte-identical file could leave both
+    // repos green having selected *different* contracts.
+    for (index, (key, _)) in pinned.iter().enumerate() {
+        assert!(
+            !pinned[..index].iter().any(|(seen, _)| seen == key),
+            "fixture defines {key:?} twice; the two repos would disagree on which wins"
+        );
+    }
     let get = |key: &str| -> &str {
         pinned
             .iter()