I was trying adding sets of strings to a set in mason.
Probably related to object identity comparison, but check on whether Parity should be using it's own compare methods where supported.
Ok, so this was intentional it seems. Set uses a custom ObjectIdentityComparator that ensures that objects are ALWAYS compared by identity. In hindsight this was probably a mistake but it's a BC issue. I'll create a new issue to change this in 2.0.