Add check for unique display names

[bradh]

What is the use case for your proposed feature? Please describe.
See #73 for the background. This is not directly user visible, it supports development.

Describe the solution you'd like
Ensure that each entry in the results of the parse has a distinct display name.

Describe alternatives you've considered
It might be possible to instantiate every entry and check the results. That could be messy in that you'd need to know what the right byte length is. Possibly it could be brute forced by trying 1,2,4 and 8 bytes.

Additional context
N/A.