This started when I was using my iMac 2013 and is still here after 'upgrading' to my Mac Mini. When ever I open LrC it opens in full screen mode on both monitors and the only way out of it is either do a force quit or Cmd+Q. I cannot swap between applications. It always goes to duel monitor. I am on LrC 11.3.1. I have uninstalled and reinstalled it. It is the only Adobe app that does this. My Ps and Lr programs run fine.