I think the word "Apostle" is a mere title. There is no doubt that Jesus is the main Apostle but, I believe there could be other representative apostles in human flesh who are representing Christ on earth.

However, they are answerable to what they did with that title. If they represent Christ well, good, and if not, they are on their own. But in all, none should place himself higher than where God placed him else, he will end up on the wrong side.