diff --git a/server/database.js b/server/database.js
index 532bd0e..924786d 100644
--- a/server/database.js
+++ b/server/database.js
@@ -96,7 +96,8 @@ export function initializeDatabase() {
       block_type TEXT NOT NULL,
       count INTEGER DEFAULT 1,
       session_start INTEGER NOT NULL,
-      last_mined INTEGER NOT NULL
+      last_mined INTEGER NOT NULL,
+      UNIQUE(turtle_id, block_type, session_start)
     )
   `);