I think if one has doubts then clearly he or she is not their soulmate. Another clue is if that person feels as passionate about you as you do about them. In order for him or her to be "the one" they would have to see you as being "the one". There has to be a mutual loving connection between you and the other person. You also must also (naturally) want the same things for the relationship and agree on how to obtain them.
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I am just trying to see the overall picture here, and share my experience. One day when I was in my late teens a young woman just wouldn't let me alone, as is often the the case at this age, wife both Men and Women. I never wondered about a soulmate.