I don't think love works quite that way. If that was true, then consider how a mother or father could love all of their children.
If this person loved another person deeply, the warning sign to me would be if they were truly over that person. I once dated a fellow who I fell madly in love with. He broke my heart, and it took a long time to get over him. He ended up marrying someone who he had known for a shorter time and had a child with her. I was deeply crushed.
I think the question to ask is, "is this person in to you?"
If the person really loved someone else, they can certainly still love you too. I would just make sure they are completely over the other person before marrying him/her, because otherwise, the relationship is not starting off on a solid ground.