fixing "global name DuplicateKeyError is not defined" error
fixing "global name DuplicateKeyError is not defined" error

mdomke/mongomock,julianhille/mongomock,chartbeat-labs/mongomock,marcinbarczynski/mongomock,akun/mongomock,drorasaf/mongomock,vmalloc/mongomock,StarfishStorage/mongomock,magaman384/mongomock