Duplicate closure only works within one site. This is by design: similar questions in different communities are likely to receive different answers. For more information, see What to do with cross-site duplicates? on the main meta.
We request that people not to ask the same question on multiple sites. In this case, the normal protocol on most sites is to close the question on all but one site; don't hesitate to flag for moderator attention. We'll generally prefer a site where the question already has answers, and migrate the question if it's been answered on multiple sites already.
If people have independently asked questions that can be considered duplicates on different sites, so be it. It's expected that the questions and the answers will be tailored to each community. Please leave a link under each question so that people can easily jump from one to the other.
The case of a question that has been plagiarized is a special one. When you see plagiarism¹, please edit to add a proper attribution if possible, and flag the post. Plagiarized posts are usually deleted but we may keep them if they are really useful.
¹ Note that Stack Exchange is a general public venue, not an academic publication. We require quotations of text, pictures, etc. to be attributed. We do not require ideas to be attributed because there is no expectation of originality here.