diff --git a/lib/bcoin/block.js b/lib/bcoin/block.js
index ecafb236..c21d16cb 100644
--- a/lib/bcoin/block.js
+++ b/lib/bcoin/block.js
@@ -232,14 +232,11 @@ Block.prototype._checkBlock = function checkBlock() {
   this.merkleTree = this._buildMerkle();
 
   // Check for duplicate tx ids
-  var unique = [];
+  var unique = {};
   for (var i = 0; i < this.txs.length; i++) {
-    if (!~unique.indexOf(this.txs[i].hash('hex'))) {
-      unique.push(this.txs[i].hash('hex'));
-    }
-  }
-  if (unique.length !== this.txs.length) {
-    return false;
+    var hash = this.txs[i].hash('hex');
+    if (unique[hash]) return false;
+    unique[hash] = true;
   }
 
   // Check to make sure block does not have more opcodes than  MAX_BLOCK_SIGOPS