"Double Falsehood" is probably based on the lost play, "Cardenio," actually by John Fletcher and mistakenly attributed also to Shakespeare. There does indeed appear to be visible evidence of "Shakespeare" in the work, but this is actually the forgery of Theobald, who, as the leading Shakespearian scholar of his time, was well equipped to add what resemble snippets from Hamlet, Lear, etc to the ms. in order to convey a notion of the authentic Shakespearian language, dramaturgy etc. Shakespeare did not plagiarize or borrow from his own works. Someone else did that.