Duplicate Signed-off-by line

I prefer my `Signed-off-by: ` line to be the first followed by `Cc:`, `Acked-by:` etc. On `stg refresh` with autosign on, I get another `Signed-off-by:` line added at the end, duplicating the first. For some reason I don't have this issue with the Python version, though I think both use `git interpret-trailers`.