[yakovdk]

If your immune system develops an antibody based on epitopes that differ in the second strain, I would think you could contract it again. Two people who overcome a virus may ultimately derive different antibodies, and some may do better at matching the class of viruses than others.

I don't think it's very common to be that unlucky in your own body's development of a suitable antibody.

At least, this is my understanding based on a recent microbiology course I took. Immune response is a fantastically complex thing.