Journal rejects my conference-to-journal extension, saying it's copied from previous publication. What should I do?
So you wrote in the cover letter that the work is an extended version of a conference paper that accompanied a poster presentation.

Apparently, this information got missing somewhere. Also this information should actually be in the paper, as Maarten van Wesel already wrote, as the later reader should be also be made aware of this.

So, judging from your post, the only thing that you did wrong was not to note this in the paper itself (please add a comment to this post if this assumption is wrong). What then went wrong in the journal's workflow is that the paper was rejected due to copying rather than asking you to add such a note in the paper. A possible explanation is that checking for plagiarism is done by the publisher's office beforehand, and they actually never saw (or read) the cover letter.

I think that it is fair to write a letter to the editor, mentioning that (1) you already wrote in the cover letter that you submitted extended work, and (2) you are of course happy to add a note that this is an extended work of [Previous work].



On a related note, there may be a small copyright problem. You may have already given an exclusive licence to distribute your work to one publisher with your earlier paper. If you now copy a good part of that work, the new publisher may see this as problematic. While many publishing agreements nowadays explicitly allow reuse of the work in extended versions, not all of them do, so the new publisher's office may suspect that this is not the case for your earlier paper. Coincidentally, many extended versions of papers are actually completely rewritten, which circumvents this problem altogether. Whether an extended version should be a full rewrite is yet another discussion, which we probably don't want to pursue in this thread.