retry failed upserts closes #4467

src/database/mongo/sorted.js

@@ -524,6 +524,13 @@ module.exports = function(db, module) {
 		data.score = parseInt(increment, 10);
 
 		db.collection('objects').findAndModify({_key: key, value: value}, {}, {$inc: data}, {new: true, upsert: true}, function(err, result) {
+			// if there is duplicate key error retry the upsert
+			// https://github.com/NodeBB/NodeBB/issues/4467
+			// https://jira.mongodb.org/browse/SERVER-14322
+			// https://docs.mongodb.org/manual/reference/command/findAndModify/#upsert-and-unique-index
+			if (err && err.message.startsWith('E11000 duplicate key error')) {
+				return module.sortedSetIncrBy(key, increment, value, callback);
+			}
 			callback(err, result && result.value ? result.value.score : null);
 		});
 	};