diff --git a/core/blockly.js b/core/blockly.js
index 558c72014..7cd0e449d 100644
--- a/core/blockly.js
+++ b/core/blockly.js
@@ -410,6 +410,10 @@ Blockly.defineBlocksWithJsonArray = function(jsonArray) {
       console.warn('Block definition #' + i +
         ' in JSON array is missing a type attribute. Skipping.');
     } else {
+      if (Blockly.Blocks[typename]) {
+        console.warn('Block definition #' + i +
+          ' in JSON array overwrites prior definition of "' + typename + '".');
+      }
       Blockly.Blocks[typename] = {
         init: Blockly.jsonInitFactory_(elem)
       };