Just make healing tips have diminishing returns if 2 ninjas on the same team has it. Reduce the 2nd ninja's buff to 20% instead of 40%. 3rd ninja 10%.

I'm not even sure how they calculate it, is it additive? or multiplicative?

No one enjoys playing 10 rounds, except the winner which 90% of the time will be the cancer team. I find it even amusing some suggest blitz because 90% of the time you will fail. There is just too much margin for error for the cancer teams.