Redundant? Yes. But it generally implies something.
"met him before" is a standard expression.
Have you met him before? - common
The main diffrence is that meet him before, usually means that you are meeting him now.
Have you met him before, meeting him this time?
Have you met him? has no suggetsion that you will meet him this time.
A: Look. There's X.
B: Have you (ever) met him? = Have you talked?
A: I met him once.
A: Look. There's X. Let me introduce you.
B: Oh, I met him before.