这是什么错误

(node:23028) UnhandledPromiseRejectionWarning: MongoError: E11000 duplicate key error collection: comment2.users index: Username_1 dup key: { Username: null }

    at Function.create (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\core\error.js:51:12)

    at toError (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\utils.js:123:22)

    at C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\operations\common_functions.js:265:39

    at handler (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\core\sdam\topology.js:913:24)

    at C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\cmap\connection_pool.js:356:13

    at handleOperationResult (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\core\sdam\server.js:492:5)

    at MessageStream.messageHandler (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\cmap\connection.js:272:5)

    at MessageStream.emit (events.js:315:20)

    at processIncomingData (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\cmap\message_stream.js:144:12)

    at MessageStream._write (C:\Users\Administrator\Desktop\作业\mongo测试\test4\node_modules\_mongodb@3.5.10@mongodb\lib\cmap\message_stream.js:42:5)

    at doWrite (_stream_writable.js:403:12)

    at writeOrBuffer (_stream_writable.js:387:5)

    at MessageStream.Writable.write (_stream_writable.js:318:11)

    at Socket.ondata (_stream_readable.js:717:22)

    at Socket.emit (events.js:315:20)

    at addChunk (_stream_readable.js:295:12)

(node:23028) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)

(node:23028) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.