Plagiarism is a rather strong accusation. It connotes a situation where one intentionally copies the work of others and represents it as one's own, without permission or attribution from the original author. Instances of outright plagiarism on Perl Monks have been few and far between in my time here, I'm happy to say (in fact I can only think of one and it's a vague memory at that). Such posts deserve to be downvoted or better yet deleted.

If I understand the OP correctly, though, it describes a situation where the essential content of a node is similar to that of previous nodes, but the poster still wrote it in his or her own words/code and where the code, particularly if it is only a one-liner, may be identical to earlier replies simply by coincidence. In that situation I don't see any good reason to downvote, and I would particularly decline to label it "plagiarism."

Update: For example, after submitting my reply I noticed that Nkuvu had replied with something similar a couple of hours ago. You can therefore argue that my reply contributed little of value to the discussion, but you absolutely cannot argue that I plagiarized Nkuvu, as the thoughts and words in this node are my own.