How to check for already existing plugin versions on activation? and prevent errors

If a user has a version of my plugin installed and activated, let's say version 1.1 And I send the user version 1.2 and he tries to upload and activate the new version, what will happen is that they will get an error: "Failed to redeclare activate_function()"

I'm struggling to see how I can prevent this. If I use class_exists or function_exists to check, that's still not working, the newer version will run first, and then the older version will error out.

I just want to do it nice and smoothly without errors. Thoughts?