It is called the Time Traveler's Paradox. In which a scientist creates a Time Machine and kills their younger self. So now a man who should not- can not- exist, somehow does. That is the Paradox, and Paradoxes are impossible.
1956
2011
1995
1996
2015
2000
1989
1986
2012
2017
2019
2021
2018
1917
1920
1933