diff --git a/src/collection.cpp b/src/collection.cpp
index 862a2c22..f8439897 100644
--- a/src/collection.cpp
+++ b/src/collection.cpp
@@ -82,6 +82,12 @@ Option<nlohmann::json> Collection::add(const std::string & json_str) {
     }
 
     std::string doc_id = document["id"];
+    Option<nlohmann::json> doc_option = get(doc_id);
+
+    // we need to check if document ID already exists before attempting to index
+    if(doc_option.ok()) {
+        return Option<nlohmann::json>(400, std::string("A document with id ") + doc_id + " already exists.");
+    }
 
     const Option<uint32_t> & index_memory_op = index_in_memory(document, seq_id);
 
diff --git a/test/collection_test.cpp b/test/collection_test.cpp
index b939792e..8f0501ca 100644
--- a/test/collection_test.cpp
+++ b/test/collection_test.cpp
@@ -1479,6 +1479,15 @@ TEST_F(CollectionTest, IndexingWithBadData) {
     ASSERT_FALSE(bad_token_ranking_field_op4.ok());
     ASSERT_STREQ("Bad JSON.", bad_token_ranking_field_op4.error().c_str());
 
+    // should return an error when a document with pre-existing id is being added
+    std::string doc = "{\"id\": \"100\", \"name\": \"foo\", \"age\": 29, \"tags\": [], \"average\": 78}";
+    Option<nlohmann::json> add_op = sample_collection->add(doc);
+    ASSERT_TRUE(add_op.ok());
+    add_op = sample_collection->add(doc);
+    ASSERT_FALSE(add_op.ok());
+    ASSERT_EQ(400, add_op.code());
+    ASSERT_STREQ("A document with id 100 already exists.", add_op.error().c_str());
+
     collectionManager.drop_collection("sample_collection");
 }