Sometimes, actors appear in a TV show early in their career, then return later for a bigger role. Other times, they’re cast in two minor roles and the showrunners just hope we won’t notice.
In-show gags like David Schwimmer playing both Ross and Russ in Friends obviously don’t count, as we’re supposed to notice this. Re-casted roles will count, but only if the early version was weird and or forgettable.
To qualify, an actor must have played two distinct roles in the same show, with the vague qualifier of people generally not noticing or realising that they’ve appeared in the very same show before.
Sometimes when these actors return, the character (or characters) they’re playing are similar enough that there might have been some off screen character progression that we just haven’t been told about.
It also counts if someone is explicitly the same character who appeared in an earlier episode, but are so different to their former version that no one remembers their origins anymore.
With all that in mind though, there’re plenty of examples of a actor/character cropping up twice in a show with no real explanation.