Being able to add a coin/banknote in multiple collections

I have had this problem from some time and i doubt im the only one.
For example, i have the “cool coins” (1950 and before coins) and the collection “Silver/billon”. What can i do if one of my cool coins is also made out of silver? I would like if I could see it when looking in both collections.

Yeah, this could be useful for some use cases. I support this.

Coins are a physical thing … in which collection is it physically? I have quite a few coins twice to fit in 2 of my collections.