At EM1 (an event at our church where the youth band multi tracked a live album) I was running monitors. Anyway the drummers (3 of them switching off) were using a metronome to keep them steady incase an overdub was needed. That was all fine, but here is where the problem came about, there were 2 sets of headphones for the drummers. One of them has some Shure earbuds he wanted to use, and the other two used some of the church's headphones. The drummers had 1 monitor mix that went through a headphone amp by the drum kit. The multiple headphone thing wasn't the issue the issue was the metronome was so loud in the headphones that during the slow/quiet parts of songs I had to turn it down in their monitor because the overhead mics picked up the metronome pretty well. Well enough for me to hear it through the FOH speakers from the monitor position. To put it short don't have unused headphones with a metronome playing loud near open mics, they go through FOH.