don't error out if the join record already exists

lib/waterline/model/lib/associationMethods/add.js

@@ -340,14 +340,24 @@ Add.prototype.createManyToMany = function(collection, attribute, pk, key, cb) {
 
       // First look up the record to ensure it doesn't exist
       collection.findOne(criteria, function(err, val) {
-        if (err || val) {
-          return next(new Error('Trying to \'.add()\' an instance which already exists!'));
+        if (err) {
+          return next(err);
         }
-        next();
+
+        next(null, val);
       });
     },
 
-    createRecord: ['validateAssociation', 'validateRecord', function(next) {
+    createRecord: ['validateAssociation', 'validateRecord', function(next, results) {
+      // If the record already exists, don't try and create it again to prevent
+      // duplicates.
+      var validateRecord = results.validateRecord;
+      if (validateRecord) {
+        return async.setImmediate(function() {
+          next();
+        });
+      }
+
       collection.create(_values, next);
     }]