git am and duplicate signatures

[Joe Perches <joe@perches.com>]

git-am -s will avoid duplicating the last signature
in a patch.

But given a developer creates a patch, send it around for
acks/other signoffs, collects signatures and then does
a git am -s on a different branch, this sort of sign-off
chain is possible:

	Signed-off-by: Original Developer <od@domain.com>
	Acked-by: Random Developer <rd@domain.com>
	Signed-off-by: Original Developer <od@domain.com>

Should there be an option to avoid duplicate signatures
in a sequence where an author can git-am the same patch?

sequencer.c:append_signoff() has a flag for APPEND_SIGNOFF_DEDUP

sequencer.c:void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag)

but

builtin/commit.c:               append_signoff(&sb, ignore_non_trailer(&sb), 0);

doesn't have an optional use mechanism available.