How to handle option conflicts?

[clagraff]

Inspired by: https://docs.python.org/dev/library/argparse.html#conflict-handler

If two options are added to the parser which share the same name, how should the conflict be handled? Currently, they will override each other such that the last option added is the option that will exist.

Is this ideal? Python allows for the option to attempt to "resolve" such conflicts. Personally, I don't think I want to support this.
I could potentially see either keeping with how the library currently overrides duplicated options; or, panic when a duplicated option name is used.