Can I master a song that's already been mastered? Yes! Every month I master songs that have already been mastered.
I prefer not too, but sometimes the client has a poorly mastered version of a song, doesn't have access to the raw mix, and wants it to sound better.
I can make the song sound "better" but if it's heavily compressed or distorted, the finished product isn't going to sound very good.
On the other hand, many times the results are surprisingly very good. You'll never know until you let me try! And, If you come to me first, you won't have to get your songs remastered!! Visit my online mastering prices page.