stgit: managing signed-off-by lines

If I import a series of patches from one of my trees into another, stgit preserves the signed-off-by lines like you would expect it to. When I export these patches it adds a signed-off-by line for myself again like you would expect it to. But adding this line on export needs to checks and make sure that my name isn't already in the signed-off-by list, without this check I have to manually remove my name when it gets duplicated.