Occasional MongoCommandException

[ball3t]

Hello all,

I am not sure if this is a problem in morphia or java HashMap. Just see if anyone encountered similar issue.
When trying to use aggregate like this:

datastore.aggregate().group(Group.group(Group.id("$xxxx")))

the following exception is thrown unless the server is restarted.

com.mongodb.MongoCommandException: Command failed with error 40324 (Location40324): 'Unrecognized pipeline stage name: '_id''

I have tried to reproduce this issue by restarting the server many times in my development server for debugging. After hitting this issue, I found the problem is that the codecs in AggregationCodecProvider is containing duplicated key. Causing it returning null when calling get with Group.class

Could any one advise on this?

Regards
** Please complete the following information: **
Server Version: 6.0.18
Driver Version: 1.19.8
Morphia Version: 2.4.4