Fix: Undefined behaviour on duplicate labels

README.md

@@ -45,7 +45,6 @@ Linux • MacOS • Windows • Docker • WebAssembly
 - ✅ [View large indexes from disk](#disk-based-indexes) without loading into RAM.
 - ✅ Space-efficient point-clouds with `uint40_t`, accommodating 4B+ size.
 - ✅ Compatible with OpenMP and custom "executors", for fine-grained control over CPU utilization.
-- ✅ Supports multiple vectors per label.
 - ✅ On-the-fly deletions.
 - ✅ [Semantic Search](#usearch--ai--multi-modal-semantic-search) and [Joins](#joins).
 

include/usearch/index_punned_dense.hpp

@@ -704,6 +704,10 @@ class index_punned_dense_gt {
 
     template <typename scalar_at>
     add_result_t add_(label_t label, scalar_at const* vector, add_config_t config, cast_t const& cast) {
+
+        if (contains(label))
+            return add_result_t{}.failed("Duplicate labels not allowed in high-level wrappers");
+
         byte_t const* vector_data = reinterpret_cast<byte_t const*>(vector);
         std::size_t vector_bytes = scalar_words_ * sizeof(scalar_at);