Preventing Duplicate Checking

[frosty tide]

So I was thinking of making it so that it checks every phantom that spawns and has a 25% chance to give it a tag that lets me make some changes to it. But normally it just keeps checking every phantom over and over again, so every phantom ends up with that tag. I heard that you can tag a mob and have any mob with that tag be ignored, but how do I ignore a certain tag? And does that method work?

[halcyon skiff]

tag=!<tag to ignore> in the target selector aeguments

[frosty tide]

Thanks! I was using != and !tag= before