I put myself in the box. I live or die. I repeat. One version of me keeps surviving (thousands of times) and becomes certain about MWI.
Versions of my colleagues on the outside see my return several times, but the chances of seeing this are the same whether MWI is right or not. So seeing me coming out of the box alive, from their point of view, wouldn't make it more likely for the MWI to be true.
Note that what I experienced in the box is nothing the outside observers couldn't imagine. They know each time that I survived, and what it looked like from my point of view.
They'd have the same information, and therefore the same premises as I, but wouldn't reach the same conclusion, even though we'd all use the same, apparently correct logic. How can this be possible? Is this something that has been discussed before?