When you set --signoff, git-duet passes the option through to Git. But in Git, that only adds a Signed-off-by for the committer. Without an author Signed-off-by, tools like PullApprove may complain about the apparent DCO gap.

In the vanilla Git case, the committer is usually the only person who can add a new signoff at that point, so I'm skeptical about landing a semantic change in Git to make git commit --signoff … add signoffs for both the author and the committer. It would be nice if Git had a --tag ... option which would append a tag (if it was missing) to the commit message, but it doesn't seem to have that at the moment. So I'm not sure what the cleanest fix for this issue is, but addressing it in git-duet seems like the right place for a duet-specific issue (i.e. “we're both sitting here and both want our signoff on the commit”).