How can we determine the winner in Love Letter if the last two players tie and have the same value of cards discarded?
I did not find the rules stated below from the English rulebook (Alderac). Maybe they did not have this situation during the early test games. The Finnish/Swedish/Danish/Norwegian rulebook (Nordic version, Lautapelit.fi), has been published a bit later, and there is a solution for this problem.



Tie-breaker for the winner of a round

The round ends if one player cannot draw a card at his turn. Then you compare the cards in your hands. If you both have the same card you will have to calculate the sum of the cards you have discarded. The rules say:


  whoever has the highest sum will win the round.


This implies that if you both have the same card in your hands and the same sum, you will both win the round (i.e., that round has multiple winners). This rule (that there might be multiple winners) is confirmed in the section concerning the beginning of a new round. It says: 


  The player who won the last round will start the next round. If the
  round had multiple winners, the youngest one will start.


But what if this happens and the game ends?

This might also lead into a (very rare) situation, when the entire game ends and two players have to equal amount of points. The rulebook of the Nordic version states:


  When a player collects the required number of points/hearts, he wins
  the game and conquers the heart of the Princess. In the case of
  draw, the youngest player wins.


So, quite thematically, the Princess prefers to fall in love with the younger guy ;) (In a real life situation, it might happen that the fate favours the one who has more digits in his bank account. But the Princess does not lack in riches so...)