Am I a terrible friend for confusing the names Evan and Aaron?

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I have two friends named Aaron and Evan. They're superficially similar, but of course I appreciate them both separately for their unique personalities. Even though I know their names very well, the wrong name occasionally slips out if I'm in a room with both of them and want to get the attention of one of them. They take great personal offense when this happens, but I argue that I'm not at fault because the names are so similar linguistically. Of course, I know nothing about linguistics but I refuse to accept that I don't know the difference between my friends. Is there any explanation for this that can help us put the issue to rest?