diff --git a/mlir/include/mlir/ExecutionEngine/SparseTensor/Storage.h b/mlir/include/mlir/ExecutionEngine/SparseTensor/Storage.h
--- a/mlir/include/mlir/ExecutionEngine/SparseTensor/Storage.h
+++ b/mlir/include/mlir/ExecutionEngine/SparseTensor/Storage.h
@@ -679,12 +679,17 @@
   /// in the argument differ from those in the current cursor.
   uint64_t lexDiff(const uint64_t *lvlCoords) const {
     const uint64_t lvlRank = getLvlRank();
-    for (uint64_t l = 0; l < lvlRank; ++l)
-      if (lvlCoords[l] > lvlCursor[l])
+    for (uint64_t l = 0; l < lvlRank; ++l) {
+      const auto crd = lvlCoords[l];
+      const auto cur = lvlCursor[l];
+      if (crd > cur || (crd == cur && !isUniqueLvl(l)))
         return l;
-      else
-        assert(lvlCoords[l] == lvlCursor[l] && "non-lexicographic insertion");
-    assert(0 && "duplicate insertion");
+      if (crd < cur) {
+        assert(false && "non-lexicographic insertion");
+        return -1u;
+      }
+    }
+    assert(false && "duplicate insertion");
     return -1u;
   }