In my actual game there was only 1 great merchant during classical era. After this great merchant was claimed the 2 remaining merchants of the classical era were skipped. During the medieval era the three were claimed. It looks that the remaining great people are skipped if one civilization reaches the next era. What I don't know is if it depends on the era of the most advanced civilization or the era of the last civilization who claimed a great person. I've found nothing on the civilopedia that's why I make this thread hopping that someone knows more than me.