I notice that if you want to persuade me away from a position, it sometimes works to have me talk with two kinds of people: 1) people who have good reasons for disagreeing with my position, and 2) people who agree with my position for similar reasons and hold it even more strongly than I do.
In both cases, the difference in opinion forces me to re-examine my reasons for believing in something, but the direction of the examination is different. Case 1 makes me think "are these criticisms valid, or do I (should I) support this position because of some reason that the criticisms do not take into account?". Case 2 makes me think "this person believes in this thing for basically the same reasons that I do, so why haven't those reasons pushed me to a similar extreme? Do I actually have unacknowledged reasons for doubting the validity of those reasons, which would deserve further consideration?"
In case 1, I am being presented with criticisms that came from outside my own thought process. In case 2, I am searching my own thought process for criticisms that have been generated within it. So the source of the criticism is either external or internal, respectively. A combination of both may prove decisive in situations where just one isn't enough.
Now the question is, are there reliable ways for inducing one of the cases in situations where only the other is present, and I have reason to suspect that I'm being overconfident about something?
I mentioned it specifically because it often does reduce my confidence in my position to encounter such a case -- or, at least it compels me to go through the process of re-evaluating my position to look for weaknesses.
I think we sometimes operate under the assumption that people who arrive at the same conclusion use the same (or very similar) reasoning. But this is not always the case.
If you encounter someone who arrives at your conclusion using different premises, then (1) his premises are faulty, (2) yours are faulty, (3) you've each found unique solid arguments or (4) you've each been sufficiently misguided, each constructed faulty premises, and both need an update.