cells emerging at a similar time, or when the queen did not find them for whatever reason, or if they did not fight. I have found three queens in a hive before, more often two. which means the bees didnt kill the other queens either in that particular case.
I have seen bees warding an emerging queen, and I have seen bees "herd" a queen away from another queen being warded, I just have never heard about the bees themselves killing un emerged queens, so its new information, and thus, exciting, need to know stuff!