I was reading Assimil Chinese with Ease and came across this.
Since this is a question, is 吗 necessary for the question? Perhaps an error? Or can you omit 吗？
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fefe is right in that omitting "吗" is allowed, but they have slightly different meanings:
你见过他？has a bit of on the rhetorical sense in it, and seems more surprised - it can be somewhat close to "You've met him, really?" or "Oh, you have ever met him?"
你见过他吗？is more of a question, as in purely asking "Have you ever met him, or not?"