here the answer is quite simple

Double blind experiments, in the context of medicine, are those in which neither the participants nor the experimenter is aware of who is receiving a treatment and who is receiving a placebo. This removes bias from both the participant and the experimenter.

here i have copied and cut short the information for your convenience:

For instance, it has been shown that a participant who believes they are receiving the real treatment will experience a measurable or perceived change, usually an improvement. The other aspect is that if the participant knows they are on the placebo, they will not experience the placebo effect So, if the participant knows what they are taking, the results for whatever is being experimentally tested for would be distorted As such, participants are not told whether they are receiving the placebo or the treatment. The second part is that the experimenter also does not know what is being given to a participant. This avoids the experimenter bias as if they know what a participant is taking However, this system is not perfect as an experimenter will generally know enough about experiment to be able to work out which participant is taking what, Overall, the double blind experiment is still the "gold standard" in experiments, as it theoretically removes all bias, treats all participants the same whether they are receiving placebo or drug and comes with a negative control.