parseopt: check for duplicate long names and numerical options

parseopt: check for duplicate long names and numerical options

We already check for duplicate short names.  Check for and report
duplicate long names and numerical options as well.

Perform the slightly expensive string duplicate check only when showing
the usage to keep the cost of normal invocations low.  t0012-help.sh
covers it.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>