Question

when finding the 90th percentile in statistics, what do i do if the highest number appears twice? for example, it appears "19, 20, 20) is 19 the 90 percentile or is 20?

Answer 1

90th percentile is the number that includes 90% of the samples
If you had 10 samples ordered small to big, the 90th percentile would be the 9th number.

Answer 2

There are slightly different ways to find the percentile, so the exact answer depends on your text book definition.