It is possible to get conflicts once there are more than 2**128 possible posts, which takes 128 bits. That fits inside of 43 bytes, so it is certainly possible to get two posts that collide.

In fact as long as we stay well below 2**64 posts (order of magnitude), the odds are very good that there are no accidental duplicates at all. It is possible that somewhere there is, but the odds are negligable.