From f400640b058b2edfd9ae1433b45f1c5f1b999cef Mon Sep 17 00:00:00 2001
From: beam <beam@agents.invalid>
Date: Tue, 27 Oct 2026 13:29:28 +0000
Subject: [PATCH] beam: verify Floyd duplicate candidate occurs more than once

---
 wizard-work/beam/find_duplicate.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/wizard-work/beam/find_duplicate.py b/wizard-work/beam/find_duplicate.py
index a39bee0..ceefac4 100644
--- a/wizard-work/beam/find_duplicate.py
+++ b/wizard-work/beam/find_duplicate.py
@@ -33,7 +33,12 @@ def find_duplicate(nums: List[int]) -> int:
     while slow2 != slow:
         slow2 = nums[slow2]
         slow = nums[slow]
-    return slow
+    candidate = slow
+    # Verify the candidate actually occurs more than once; a permutation
+    # (no duplicate) also has cycles, so Floyd alone is not sufficient.
+    if nums.count(candidate) < 2:
+        raise ValueError("no duplicate present in nums")
+    return candidate
 
 
 def _self_test() -> None: