This message, ‘Updates were rejected because the remote contains work that you do’, occurs because there are new commits in 10gen/mongo.
1: new commit1 come in
2: copybara1 triggered and start to run
3: new commit2 come in
4: copybara2 triggered  and start to run
and etc
copybara1 synced both commit1 and commit2 to mongodb/mongo. copybara2 returned the the message with “‘Updates were rejected because the remote contains work that you do’,”
I will include this message in our list of acceptable error messages for the copybara task