Changing Key of section - confused

I'd completed part of a score before I realised that there had been a key change. I tried to drag the correct key symbol to the correct bar. It seemed to break stuff. The key symbols didn't appear properly. I wanted one sharp but I got one sharp and one natural (!). I don't really mind if I have to go back through and fix the accidentals, but how do I make it add the key signature that I actually want??

Version 0.9.5 by the way, and any help appreciated.