diff --git a/bigchaindb/backend/mongodb/schema.py b/bigchaindb/backend/mongodb/schema.py
index 01eac29c..572acff9 100644
--- a/bigchaindb/backend/mongodb/schema.py
+++ b/bigchaindb/backend/mongodb/schema.py
@@ -50,9 +50,10 @@ def drop_database(conn, dbname):
 def create_bigchain_secondary_index(conn, dbname):
     logger.info('Create `bigchain` secondary index.')
 
-    # secondary index on block id which is should be unique
+    # secondary index on block id which should be unique
     conn.conn[dbname]['bigchain'].create_index('id',
-                                               name='block_id')
+                                               name='block_id',
+                                               unique=True)
 
     # to order blocks by timestamp
     conn.conn[dbname]['bigchain'].create_index([('block.timestamp',
diff --git a/tests/backend/mongodb/test_queries.py b/tests/backend/mongodb/test_queries.py
index 0fd7229a..3ea7db28 100644
--- a/tests/backend/mongodb/test_queries.py
+++ b/tests/backend/mongodb/test_queries.py
@@ -299,12 +299,13 @@ def test_count_blocks(signed_create_tx):
     from bigchaindb.models import Block
     conn = connect()
 
+    assert query.count_blocks(conn) == 0
+
     # create and insert some blocks
     block = Block(transactions=[signed_create_tx])
     conn.db.bigchain.insert_one(block.to_dict())
-    conn.db.bigchain.insert_one(block.to_dict())
 
-    assert query.count_blocks(conn) == 2
+    assert query.count_blocks(conn) == 1
 
 
 def test_count_backlog(signed_create_tx):