1 files changed, 9 insertions, 1 deletions
diff --git a/src/dynamic.c b/src/dynamic.c
index 5915721..6a1dae8 100755
--- a/src/dynamic.c
+++ b/src/dynamic.c
@@ -317,8 +317,16 @@ Return value:
         {
             ctx->mvKiller[uPly][1] = ctx->mvKiller[uPly][0];
             ctx->mvKiller[uPly][0] = mv;
-            if (ctx->mvKiller[uPly][1].uMove == 0)
+            if ((ctx->mvKiller[uPly][1].uMove == 0) &&
+                (!IS_SAME_MOVE(ctx->mvNullmoveQuietRefutations[uPly], mv)))
             {
+                // Guard against backfilling slot[1] with the same move
+                // just placed in slot[0] -- e.g. this ply's recorded
+                // null-move quiet refutation happens to coincide with
+                // the move causing the current fail-high. Without this
+                // check both slots would hold the identical move,
+                // wasting a killer slot (and, in a DEBUG build,
+                // tripping the assert below) for no ordering benefit.
                 ctx->mvKiller[uPly][1] = ctx->mvNullmoveQuietRefutations[uPly];
             }
         }