Can you do a PhD solely on repeating experiments of others? In here, they say there is a problem with results not being replicated. I know a PhD is supposed to be "original" research. But could you do a PhD solely on replicating the results of others? Especially given that there is a problem with results not being replicated, I believe you would be contributing a lot to science. Even if it is not "original research".
I would say it depends:

a) If in medicine, you are the first one to replicate an effect in an independent study, then you actually did something original, namely confirming the effect

b) If in physics, you refine an experimental design to verify an uncertain outcome, or clean up a measurement to exclude artifacts, and make the effect more clear, it is also something original

The rest is in the shades of what passed in the journals as "original". But I guess as long as the experiment was set up freshly (i.e. not in the same group) and had any small improvement/change (analysis method, amount of data, statistical uncertainty, control experiments) over the original it would pass in many universities for a PhD (and I will not give my personal opinion on the level or originality required for a PhD in an average university here).