Seq.duplicate() isn't thread safe #122
Description
As correctly stated by Tagir Valeev, the current implementation of Seq.duplicate() isn't thread safe:
http://stackoverflow.com/questions/23860533/copy-a-stream-to-avoid-stream-has-already-been-operated-upon-or-closed-java-8/25961951?noredirect=1#comment53095315_25961951
This is relevant when the stream duplicates are consumed on different threads, which is (ignoring some edge cases) the only reasonable use-case for duplication AND consumption at equal rate.