Insufficient repetition is a huge problem in medical science. There is no incentive to replicate existing studies, as it doesn't advance your career. A related problem is that there is no incentive to publish negative results.
The widespread non-publication of negative studies (and, tragically, their datasets) has, rightly, tarnished the validity of all published scientific results.
You can't blame normal people for doubting "scientific" results. It doesn't take a rocket scientist to deduce the worthlessness of published results, when it is disclosed that negative results have been withheld:
Just do studies until you get the results you want, then publish that one.
This has been de rigeur in medical science for some time.
For shame...
I say in theory because this would cost a rather large fortune and would probably be illegal in most nations, so it's not likely to actually happen.
Which sounds pretty similar to "science is broken", from an outsider's perspective.