what happen the second time you play the same unique card.For example i have already steward of gondor on frodo amd i want to play another copy on sam. Do i discard the first and place the second on sam...or it just impossible to play a second copy of a unique.

When an unique card with the same name is already in play, the other card fail to come into play and stay whereever it was.

That's right. You cannot have two identical cards in play at the same time, just as two players cannot summon Gandalf in the same turn.

if a card fail to come in play due to unique card rule. What happen to ressources and the second card ?

ressources lost card in discard pile or just simply you didnt play it at all ?

In the rulebook : the unique cards fails to enter play and remain where it was. Don't know about ressources, but either you wanted to play it, and you couldn't... I don't know why you would do that in the first place... or it came into play with another cards, and the effect of the cards fails, so the cards is still played and paid as normal.