so have collection has documents this: 
now, documents have same 'dmca.id' field (this not id assigned mongodb), i.e. duplicates, want remove them , keep 1 copy. there more 4 million documents, want fastest way possible. , mongodb version 3.0.2. saw post removed duplicates valid version 2.x . the root object name need not dmca always, can else well.
{"dmca":{"id":407,"type":"dmca","title":"dmca (copyright) complaint google","body":null,"date_ {"dmca":{"id":408,"type":"dmca","title":"bpi dmca (copyright) complaint google","body":null,"d {"dmca":{"id":409,"type":"dmca","title":"music dmca (copyright) complaint google","body":null, {"dmca":{"id":410,"type":"dmca","title":"dmca (copyright) complaint google","body":null,"date_ {"dmca":{"id":411,"type":"dmca","title":"stealth v. stealth signal","body":"dear president:\r\n\r
