Posting someone else's words as if they were one's own without acknowledgement, is indeed plagiarism, whether it is copyright infringement or not, and should not be tolerated here. To that extent I agree with the answer by Ryan M.
However, when I was called on Wikipedia to deal with a suspected case of copyright infringement or plagiarism by an editor there, fairly often it turned out that some other site had copied from Wikipedia without acknowledgement, not the reverse. It is perfectly possible that someone copies a question from this site and reposted it on some other site or forum, without acknowledgement In that case the poster here has done nothing improper. In fact, I see several possible cases when a question (or answer) is duplicated here and on another site, assuming in all cases that the post is otherwise on-topic and appropriate here.
The poster on law.se and on the other site are or seem to be the same person. In that case, the poster should be pointed at the policy on external cross-posting, and the post here should be edited to declare that it has been cross-posted, with a link to the other version.
The post here seems to have been copied by a poster on another forum, who is apparently not the poster here. The post here should be edited as in case 1 to disclose the cross-post, but there is no need to notify the poster here because that poster has done nothing wrong.
The post here seems to have been copied without proper acknowledgement from another place, by someone who was not the original author. That is plagiarism, and perhaps copyright infringement as well. The poster should be warned not to do that again, and the post should either be edited to acknowledge the source, with all quoted content marked as such using quote marks or block-quote syntax; or else it should be deleted here. In either case the moderators should be notified via a flag so that they are aware of the situation, and can suspend the user or take other measures should the same user repeat such action.
The post was an external cross-post in compliance with policy, including the presence of a link to the other venue.
The post here is a copy of a post from another venue, posted here with proper acknowledgement including a link, and using quote syntax to indicate copied text. The poster here is apparently not the same as the original poster on the other site. The poster here should be pointed to this discussion and any relevant policy. The community should discuss on meta, or possible the mods in a mod chat, whether to delete the post here. If the original post was under a CC license, or some other free or permissive license, that should IMO weigh toward keeping the version here.
If a user here sees a post that duplicates (in whole or in significant part) a post on another site, without acknowledgement but is not sure which of cases 1-3 above it fits, it would be a good idea to flag it for the mods, along with as much reliable info as the user has available.