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Plagiarism shouldn’t lead to getting red tagged, but rather banned.
As to the question you’re asking, it’s always way better to prevent a post from being reported by adding the source, even if it is to a post made by the same person on another media.
I do recall a case or two where the person being reported for plagiarism has actually copied it from the media (using a non-coincidental user name). Proving ownership is easy by editing the something in the original source to link it to the Bitcointalk account, and that is what I remember a couple of those cases doing. Nevertheless, it’s better to prevent incurring in needing to provide evidences and such, simply by placing the link (and perhaps even mentioning that it’s the same person).