diff --git a/src/sprs.c b/src/sprs.c
index 99009c6..5939e0c 100644
--- a/src/sprs.c
+++ b/src/sprs.c
@@ -32,9 +32,13 @@ void sprs_free(sprs* sprs){
 }
 void sprs_put(sprs* sprs, size_t index, void* data){
   if(index < sprs->capacity && sprs->size < sprs->capacity){
-    sprs->dense[sprs->size++] = index;
-    sprs->sparse[index].index = sprs->size - 1;
-    sprs->sparse[index].data = data;
+    if(!sprs_contains(sprs, index)){
+      sprs->dense[sprs->size++] = index;
+      sprs->sparse[index].index = sprs->size - 1;
+      sprs->sparse[index].data = data;
+    } else {
+      sprs->sparse[index].data = data;
+    }
   }
 }
 int sprs_contains(sprs* sprs, size_t index){