See this:
0.0 20.0
5
0.0 16.0
1.0 17.0
2.0 18.0
3.0 19.0
4.0 20.0

The output should be any ONE of the five

You should try to keep the list short. That means, only observers covered all the time are needed, which are no more than 2 each moment indeed, and others should be deleted.