How do scientists repeat experiments already done by someone else?

They try to measure the same circumstances as the original study in the same way. So, for instance if the original study was trying to figure out if cell phones were harmful, and the original study tested cell phones on rats, they would get a bunch of rats and expose one group to cell phone signals all day, and the other group not at all, and see if they got the same results as the first people who did that... they are basically checking the work of the first scientists, and seeing if the same thing happens every time.