How do you file a second amended return for the same tax year

I already filed an amended 2014 tax return. However I just realized that I need to file an second amended return for 2014. How do I do that? When I put the information in turbo tax it doesn't seem like it recognizes the first amended return. It looks like it is making the adjustment against my original 2014 return and not my adjusted 2014 return. Can you please advise?