Gave up trying to decipher the existing logic for safe ways to
de-duplicate manually and switched to an `IndexSet` to enforce only one
of each argument exists.

Fixes clap-rs#3556

This makes it consistent with other errors where we show arguments to
the user.

With us moving the required de-duplication up a level, it made this
check redundant.  By removing this check, we're more likely to have an
item in the `incls` which forces a smart usage and reduces the chance of
an `[ARGS]` or `[OPTIONS]`, so a couple of tests changed.