When this card is in play for the Redhorn Gate expension, can i have another arwen ally card in my deck but just not playing it?

Absolutely, you can have Arwen in your deck but can't play her or put her inot play as you said since she is unique. The Arwen objective card is also unique and shares the exactly same name as the ally which is the key.

Then what will happen when i use Elrond with his ring to summon the top card arwen of my deck for free?

Cutievalkyrie said:

she cannot be played legally, so she goes to the bottom of your deck.