It depends what you mean precisely.
The overall chance of opening the same mythic you got in a previous box (or any other given mythic) in a future box increases as the overall number of boxes you open increases.
But the chance of getting the same mythic in BB #2 that you got in BB #1 is identical to the chance of getting the same mythic in BB #3 (or any other box) that you got in BB #1.