Fetching in parallel from a remote group with a duplicated remote results
in the following:
error: cannot lock ref '<ref>': is at <oid> but expected <oid>
This doesn't happen in serial since fetching from the same remote that
has already been fetched from is a noop. Therefore, remove any duplicated
remotes after remote groups are parsed.
Signed-off-by: Calvin Wan <calvinwan@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>