Given the current difficulty, I think the first 64 bits or so of block hashes are all zeros. As difficulty increases, the number of effective bits in the valid hash are reduced. Therefore, the odds (assuming difficulty doesn't increase beyond the effective difficulty of the current block) that any valid future block will have the same hash as the current block is about 2¹⁹², not 2²⁵⁶.  The birthday paradox increases the likelihood that there will be two blocks that match each other (rather than a block that matches a specific block).

Of course, even 2¹⁹² is a ridiculously large number.

So, we are discussing something that most of us agree isn't going to happen. Even if the OP can't wrap their mind around just how unlikely it is, I'm not yet convinced that a duplicate block hash would be a serious problem anyhow.  So not only are we discussing something that isn't going to happen, it might not even be a problem if it was possible.