An experiment can be repeated in respect to the factors one considers significant, beforehand, so a repetition at another time and place is only provisionally the same. Another way of doubting is to ask how many repetitions of an experiment are necesary to prove a cause and effect relationship.

All of science depends on the idea of cause and effect, but that principle itself can't be tested, since you would have to start at the beginning of time and test until the end.

   The ancient Greeks used paralax to test whether the earth moved in space, and found that it didn't. It was a valid experiment. The shifts were too tiny, and the Greeks could not imagine themselves so tiny in a universe so vast.

It's fun to doubt.