diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
new file mode 100644
index 0000000..2340abd
--- /dev/null
+++ b/.pre-commit-config.yaml
@@ -0,0 +1,4 @@
+-   repo: git://github.com/pre-commit/pre-commit-hooks
+    sha: '96fb7fa10f2f4c11ed33482a9ad7474251e5e97f'  # Use the sha you want to point at
+    hooks:
+    -   id: check-json
diff --git a/jaccard.js b/jaccard.js
index d1a35a8..9f1c13e 100644
--- a/jaccard.js
+++ b/jaccard.js
@@ -11,7 +11,7 @@ var async = require('async');
 var intersection = function (a, b, c) {
   var x = [];
   var check = function (e, cb) {
-    if (~b.indexOf(e)) x.push(e);
+    if (~b.indexOf(e) && !~x.indexOf(e)) x.push(e);
     if (cb && typeof cb == 'function') cb(null);
   };