[{_id:123,Customer:"customer A",State:"TX",City:"Houston",checksum:1111,IsBundlled:false},{_id:1234,Customer:"customer B",State:"TX",City:"Houston",checksum:1111,IsBundlled:false},{_id:12345,Customer:"customer C",State:"NY",City:"Buffalo",checksum:2222,IsBundlled:false},{_id:123456,Customer:"customer D",State:"NY",City:"Buffalo",checksum:2222,IsBundlled:false},{_id:789,Customer:"customer E",State:"NY",City:"Buffalo",checksum:3333,IsBundlled:false}]

db.collection.aggregate([{$group:{_id:"$checksum",_ids:{$push:"$_id"}}},{$match:{$expr:{$gt:[{$size:"$_ids"},1]}}},{$group:{_id:null,checksums:{$push:"$_id"}}}])