Never say never = You do not know the future, so you can not say you will not do something because you may. For example, I used to say that I would never date a coworker. It turns out that I am with a coworker now!

keep an open mind and do not exclude any options that might later turn out to be possible without you currently be aware of it

The irony of the phrase is that it uses self-contradictory logic, in that you just said "never" twice.

there is a song: Don't Give Up - Peter Gabriel and Kate Bush