SG cannot recognize some of the games I already own in library

Did you buy them recently? If so, then try re-syncing your account here on Steamgifts.

No. Re-sync doesn't work. They've been in my library for a very long time.

Think its because OMD and OMD Goty have seperate app ids. You can use the hide button so you wont see the giveaways.

Same problem here.
The only "solution" I ended up with was to click on the crossed eye just after the game name on SG, so I don't see these games anymore.

Still not solving SG problems with dishonest users who can enter giveaway for games they already own :/

It is probably a steam Api issue.

This is a steam api issue. See this thread

Edit: well I know Dead Rising is an issue. OMD is because of different App ID, possibly Sniper too.