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Usually when two co-workers have a problem, then they will just hate each other and try to be better than the other. You can't say this because it is the wrong way to handle it. I would probably not deal with the problem and hate the other person, but that is not what I would say in an interview. You can mention that you believe having to deal with a problem with a co-worker is difficult because competitiveness comes into factor. Along with this statement, follow up with how you would try to look beyond that to make your relationship better.