format-patch -s shouldnt sign 2nd time

Ive started to manually add SOB
since it needs to be above the
---, followed by version info.

git format-patch -s ...
will sign again, and then
checkpatch will complain

WARNING: Duplicate signature
#178:
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>

A selective addition would simplify this situation.