shortparam-659269: Add checks for duplicate short params
shortparam-659269: Add checks for duplicate short params

You can't have 2 parameters with the same short version. This enforces
that check.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

mozilla/gcli,joewalker/gcli,joewalker/gcli,mozilla/gcli,mozilla/gcli