Traversing the vertices in order around the perimeter, you visit each one only once, so each vertex identifier will be used once.

_____

One can conceive of polygon shapes that have two (or more) vertices on top of each other, so that a given letter could be used more than once.

_____

One can conceive of polygon shapes that have two (or more) vertices on top of each other, so that a given letter could be used more than once.