As explained by Richa Mishra, the phenomenon is called "Deja Vu". The feeling that something has happened to me already when you're experiencing it for the first time.
Deja Vu occurs when your brain does something called "cognitive expectancy", which evolved with humans as a part of evolution. It helped them in hunting and other chores. It showed them an expected future (of very short duration) based on present actions, much like a complex mathematical formula. Rest assured, it has no side effects and it is completely normal.
Thanks for asking.