When does Unity create the Info.plist file for iOS builds?

I need to add some data to our plist file for iOS builds so I created a post build process to do it. I just insert some xml into the existing file. I noticed however that sometimes I end up with multiple copies of my data in Info.plist, so it seems like Unity doesn't rebuild that file every time you trigger a build.

Does anyone know what causes Unity to rebuild this file and how I might be able to trigger off of this event, since just doing it every build gives me this problem?

I know I can always fall back on checking to see if my data is in there first, but I thought I'd see if there was a better solution out there.