Is there a mod i can use to flag games i already own on GOG and EGS?

I just use the GOG Galaxy client with all the integrations set up properly, that way I can check if I own the game anywhere and not waste entries like that

I misuse Steam shop's "not interested (played on another platform)" button.

Playnite? I’ve never used it, but some people love it.

Good question, following

I track everything I own across all platforms with Playnite (I never launch stuff with it; just use it as a catalog). Then I can search there before entering giveaways.

ESGST has categories where you can set up your own categories . i use this to add tags to games i already own on GOG/EPIC or do the opposite by tagging games im interested in without wishlisting everything. but you need to go through every game induvidually

I export the Steam IDs of all the games that I own on other platforms to a text file from Playnite. I then use that list to mass ignore/hide those games on Steam and SG using this userscript and ESGST.

It's a pretty convoluted process, but I only have to do it once every 1-2 months.

Sharing my process - I have everything on Playnite. The first time I synced all my libraries there, I made a filter to show every game except the Steam ones. Then I ended up going one by one to their Steam page and ignoring them there.

ESGST has an option to hide "Ignored" games on SG. I also set ESGST to add a red flag / tag for Ignored games, so in case I'm navigating a train and see it there, the red tag reminds me I shouldn't join. In these cases I also manualy hide it here on SG

The first time setting this thing up will consume a lot of time, since I assume you have many games on EGS and GOG you want to manually Ignore on Steam. But once the setup is done, the only thing you need to remember is to manually Ignore on Steam new games you might get on another store. It's easy to maintain once you've set it up

If I ever get to the point where this becomes an issue I'd probably reconsider some of my hoarding habits 😅