I see the distribution of games that I already have

There is a paragraph for this in the FAQ:

What should I do if I win a game more than once, or if I win a game that I already purchased?
When this occurs, you should contact the giveaway creator and inform them you will not be accepting or redeeming the gift. Explain the situation, and ask the giveaway creator to request a new giveaway winner. During this process you should not mark the gift as "Received", or reveal the key if the option is available. This will help inform us that you are not accepting the gift, and it will prevent you from being suspended for not activating a gift you won. Once the ticket from the giveaway creator is approved for a new winner, the win will be removed from your profile.

Some game and DLC that also included in Gold edition, Collection edition,... of that game on steam will have a different steam ID. This mean steamgifts may not hide those game even if you already have it. In case you win such giveaway, just ask the giveaway creator to reroll.

ENG:
Steam API doesn't show if you own a DLC to SteamGifts. Better to check your Steam library manually before entering anything. Hiding base games should work reliably though.

RUS:
API Steam не показывает, есть ли у вас DLC для SteamGifts. Лучше проверить свою библиотеку Steam вручную, прежде чем что-либо вводить. Однако скрытие базовых игр должно работать надежно.

Easy. u won a game/dlc u already have? leave a nice comment about reroll.
just read description and check every dlc/pack u enter . u have it - hide it.

Удалённые из стима игры, DLC и наборы игр (всякие Complete Edition) часто не детектируются сайтом корректно.
Если уж так случилось, что выиграл игру, а она уже есть - надо не трогать ключ и не отмечать раздачу как полученную, а вместо этого написать комментарий в раздаче с просьбой к раздающему сделать ре-ролл.
Я когда вижу такие игры - заношу в игнор-лист, чтобы мне они больше не отображались.