Every time I open my SU model, my OS tells me it is already open. “Do you want to open as Read Only?” This only happens with my skp file, no matter how careful I am about my clicks. Anyone have advice??
Win11
usually this happens when you double click too much on a file 
maybe the opening was slow, maybe you did a triple click, and SU considers you’re trying to open the file twice. one will open in the background, the second will tell you it’s read only.
when this happens, you could check at the bottom of your screen the sketchup icon. if you place your mouse on it, there might be 2 or more windows opened, one with the problem, the other not.
Plan B, quite sketchup (completely), and don’t double click on a specific file. instead, launch sketchup, and from there open the file you want. this should prevent a multiple opening.