Woot a question! In the final write-up for the setting the number of different stances a character can use at one time is more limited (a single stance except for martial artists which can ultimately use up to four at a time by 17th level), but one stance feat taken two or three times still only qualifies as one stance (so having Stance of Broken Stone three times doesn't provide +3 to AC while unarmored, just +1 to AC). Essentially you've got one stances slot and multiple copies fit into that slot just fine, it's only a problem if you know more than one martial arts stance.