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(Apologies if my terminology is inexact, I hope people can get the drift of my question regardless of the detail). Assume there are 2 parties, say A and B, unknown to each other. A files a provisional patent and gets a priority date. Some time later B files a provisional patent covering much the same invention. After a year, A allows the provisional to lapse. A and B then meet up and B learns about the content of A's provisional application. B wishes to continue and pursue a patent filing. My question is this: Does the fact that A has documented evidence of prior knowledge of the invention (even though this is not in the public domain) mean that the novelty of B's filing can be challenged (by A, for example)?