Even just a minute ago, the telling is past, and you have now, as you speak, indeed heard of it. Therefore, use past perfect if the main clause is in the past However, if you are actually speaking to him at that moment, have is permissable and probably common; the conversation is not yet finished, you see-- you might change your mind as the conversation continues, and say something like ' oh yes, I have already heard of that after all' .
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