Missed edge case: if all three numbers are equal

Realized my old code didn't handle this, while reading a Stack Overflow
thread.
Compute area, perimeter, diagonals of rectangle

For the purpose of this exercise, I blindly trust that the user will
read this and supply a guaranteed rectangle, in one of two ways:
- As lengths of two adjacent edges or
- As a set of adjacent coordinates.

Guaranteeing correctness of a rectangle is proving to be rather
cumbersome as I can only construct pairs and access elements of pairs,
at this time.