Should I put my supervisor as coauthor of a paper? I was working on a problem in the field engineering. I have used one approach and it turned out that it does not work well. After that I got into conflict with my supervisor, and realized that it will be hard to publish the original paper with him. So, I have redone everything with another approach, wrote a paper, submitted it and at as soon as my paper have been almost accepted, my supervisor wrote to the editor claiming that he also should be an author of the paper. He haven't even seen the new paper nor haven't analysed or interpreted the new results. What should I do in this situation?
This entire situation is an ethical quagmire that both you and your advisor are responsible for.

The reason why I say that you share in the creation of this situation is the following. Let's replace your advisor with an external collaborator. The sequence of events, as you have outlined here, runs as follows:


The two of you collaborated on an initial version of the paper.
You had a conflict on some issue (seemingly related to this paper)
You then proceeded to go behind the collaborator's back, redo the analysis, rewrite the paper, and submit it without informing her.


Under such circumstances, it is clear that you would bear a large percentage of the blame for the situation. The fact that it's your advisor instead of an external collaborator doesn't change the ethical considerations here.

Your advisor probably feels that because you were previously working on the problem together, you have cut him out of the loop without his consent. You should have showed him the preliminary results of your new analysis and let him decide if he wanted to collaborate further. If he was to be an author on the original paper, he was entitled to at least that much.

Of course, at this stage, it's hard to say what to do—your advisor has also complicated the situation by asserting his author rights on a paper he allegedly has never seen, which is also wrong. I would follow JeffE's advice here and take the path of least resistance. Getting out of this mess of a relationship is the critical step right now.