Now that the characters are more developed and have a certain history, the stories can incorporate more emotion thus making the entire show more intense and emotionally involved.
I would react in a calm manner and "walk away". In my case this means never forgetting and maybe forgiving but the relationship will never be the same again. I will in my own way ensure that this person feels guilty as long as I am alive and never forget that s/he is a loser.
Yes, as long as the person is right for me in the ways that I feel is important, dating history would not be important to me.
If you are conscious about this, however, you can always keep it to yourself and see how the relationship unfolds and share when the opportunity presents itself. It is...