Fix .createEach() bug. (closes balderdashy/sails#7266)

lib/waterline/utils/query/forge-stage-two-query.js

@@ -1308,6 +1308,24 @@ module.exports = function forgeStageTwoQuery(query, orm) {
       throw buildUsageError('E_NOOP', 'No things to create were provided.', query.using);
     }//-•
 
+    // Ensure no two items in the `newRecords` array point to the same object reference.
+    // Why?  Multiple references to the same object can get tangly and cause problems downstream
+    // in Waterline, such as this confusing error message: https://github.com/balderdashy/sails/issues/7266
+    //
+    // On the other hand, simply using `.uniq()` to deduplicate can be somewhat unexpected behavior.
+    // (Imagine using `let x = {}; await Widget.createEach([x,x,x,x]);` to create four widgets.
+    // It would be a surprise if it only created one widget.)
+    if (query.newRecords.length !== _.uniq(query.newRecords).length) {
+      throw buildUsageError(
+        'E_INVALID_NEW_RECORDS',
+        'Two or more of the items in the provided array of new records are actually references '+
+        'to the same JavaScript object (`.createEach(x,y,x)`).  This is too ambiguous, since it '+
+        'could mean creating much more or much less data than intended.  Instead, pass in distinct '+
+        'dictionaries for each new record you would like to create (`createEach.({},{},x,y,z)`).',
+        query.using
+      );
+    }//-•
+
     // Validate and normalize each new record in the provided array.
     query.newRecords = _.map(query.newRecords, function (newRecord){