How do I know if I've already befriended a Yo-kai? (Yo-kai Watch 2)
There really isn't a clear path to this from my experience. You can only hold a maximum of 6 of each type meaning you can hold but the higher number you have the less you will see these types. It also does not scale meaning that by the time you get to your 5th and 6th its not a stronger Yo-kai because you have duplicates of this. It seems like a huge design flaw overall. I had originally used this article when I wondered the same.
http://segmentnext.com/2015/11/08/yo-kai-watch-how-to-befriend-yo-kai-manage-medals-best-yo-kai-order/