Thanks for replying! Only one session. Running systemctl status as root gave me this result. I could only find one /usr/bin/kwin_x11 --replace (ID: 4426) entry, though when I can see many others: 4426, 4427, 4429, 4430, 4433, 4434, 4436, 4442, 4443, 4444, 4445, 5871 10277, 10278, 10279, 12222, 12223, 12224, 12225, 12226, 12227 and 12228.
You know I bet those are just the threads kwin is using, since memory usage is all the same. Linux threads are just processes that share everything. You may be able to look at your top (htop?) Options to show threads separately or together?