diff --git a/src/cmc/treemap.h b/src/cmc/treemap.h
index 0ce0bff..08bd869 100644
--- a/src/cmc/treemap.h
+++ b/src/cmc/treemap.h
@@ -415,7 +415,10 @@ static const char *cmc_string_fmt_treemap = "struct %s<%s, %s> "
                 else if (_map_->f_key->cmp(scan->key, key) < 0)                \
                     scan = scan->right;                                        \
                 else                                                           \
+                {                                                              \
+                    _map_->flag = cmc_flags.DUPLICATE;                         \
                     return false;                                              \
+                }                                                              \
             }                                                                  \
                                                                                \
             struct SNAME##_node *node;                                         \
diff --git a/src/cmc/treeset.h b/src/cmc/treeset.h
index a9ac785..796a050 100644
--- a/src/cmc/treeset.h
+++ b/src/cmc/treeset.h
@@ -397,7 +397,10 @@ static const char *cmc_string_fmt_treeset = "struct %s<%s> "
                 else if (_set_->f_val->cmp(scan->value, element) < 0)          \
                     scan = scan->right;                                        \
                 else                                                           \
+                {                                                              \
+                    _set_->flag = cmc_flags.DUPLICATE;                         \
                     return false;                                              \
+                }                                                              \
             }                                                                  \
                                                                                \
             struct SNAME##_node *node;                                         \
diff --git a/tests/main/src/treemap.c b/tests/main/src/treemap.c
index c7e6c3d..6034d58 100644
--- a/tests/main/src/treemap.c
+++ b/tests/main/src/treemap.c
@@ -237,7 +237,10 @@ _Bool tm_insert(struct treemap *_map_, size_t key, size_t value)
             else if (_map_->f_key->cmp(scan->key, key) < 0)
                 scan = scan->right;
             else
+            {
+                _map_->flag = cmc_flags.DUPLICATE;
                 return 0;
+            }
         }
         struct treemap_node *node;
         if (_map_->f_key->cmp(parent->key, key) > 0)
diff --git a/tests/main/src/treeset.c b/tests/main/src/treeset.c
index 8aa44f9..b21489c 100644
--- a/tests/main/src/treeset.c
+++ b/tests/main/src/treeset.c
@@ -229,7 +229,10 @@ _Bool ts_insert(struct treeset *_set_, size_t element)
             else if (_set_->f_val->cmp(scan->value, element) < 0)
                 scan = scan->right;
             else
+            {
+                _set_->flag = cmc_flags.DUPLICATE;
                 return 0;
+            }
         }
         struct treeset_node *node;
         if (_set_->f_val->cmp(parent->value, element) > 0)
diff --git a/tests/main/unt/treemap.c b/tests/main/unt/treemap.c
index 519c955..313872d 100644
--- a/tests/main/unt/treemap.c
+++ b/tests/main/unt/treemap.c
@@ -64,6 +64,9 @@ CMC_CREATE_UNIT(treemap_test, true, {
         cmc_assert(tm_insert(map, 1, 1));
         cmc_assert_equals(int32_t, cmc_flags.OK, tm_flag(map));
 
+        cmc_assert(!tm_insert(map, 1, 2));
+        cmc_assert_equals(int32_t, cmc_flags.DUPLICATE, tm_flag(map));
+
         // update
         cmc_assert(!tm_update(map, 2, 2, NULL));
         cmc_assert_equals(int32_t, cmc_flags.NOT_FOUND, tm_flag(map));
diff --git a/tests/main/unt/treeset.c b/tests/main/unt/treeset.c
index 62a61ff..93a98ed 100644
--- a/tests/main/unt/treeset.c
+++ b/tests/main/unt/treeset.c
@@ -56,6 +56,9 @@ CMC_CREATE_UNIT(treeset_test, true, {
         cmc_assert(ts_insert(set, 1));
         cmc_assert_equals(int32_t, cmc_flags.OK, ts_flag(set));
 
+        cmc_assert(!ts_insert(set, 1));
+        cmc_assert_equals(int32_t, cmc_flags.DUPLICATE, ts_flag(set));
+
         // remove
         cmc_assert(!ts_remove(set, 2));
         cmc_assert_equals(int32_t, cmc_flags.NOT_FOUND, ts_flag(set));