After an update to high sierra most program icons look the same.
Any ideas how to fix that?
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Do you have League of Legends installed on your Mac ? It seems to be the cause of the issue for many other mac players.
Unfortunately there is nothing you can do until Riot acknowledged the problem and deploy a fix.
Eventually this is a MacOS software issue. You need to boot the mac into recovery mode (Turn Mac off, then while turning it on, you press and hold Command + R and hold it until you see an Apple logo) and reinstall MacOs to fix the icons. It wont delete any information since its only rebuilding the system files. That should fix the icons.
Try rebuilding the Spotlight index.
This article from Apple will tell you how: https://support.apple.com/en-gb/HT201716