It's the same deal for "met" and "heard", so to keep it simple I'll just use "met" in the examples. "I met him" means that I'm talking about a specific time in the past when I met him "I have met him" means that I've met him on one or more occasions on the past. It's not important when those occasions were; all I'm trying to get across is that I have met him at some time.
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