I've DMed a game where there were 4 summoners with rings of invisibilityReally? Why on earth would 4 of 5 players choose to play the same class, with the same magic item?
I told them, "The general consensus on the Forums is that Summoners are OP. So 4 of you play summoners, and one of you play a cleric that focuses entirely on healing since you only have to be within 100 feet of the battle.
It was me wanting to see what all the whining was about. A balanced power-party would have been much, much scarier.