(Duplicate of this question since I don't have enough rep to add a comment).
Essentially when using Search and using the page token to get more results, the results in the following pages tend to have results from the previous pages. The more pages you go through, the more and more repeating videos appear.
You can test this directly via the documentation which allows you to perform calls from there. Do a search query for anything, keep track of the video IDs in the results, wait a few seconds, and then do another query with the next page token, and repeat. It sometimes takes around 5 or so pages before a duplicate shows up. The same issue happens if you search for related videos instead of a query.
Is this intended behavior? I cannot seem to locate anything in the documentation mentioning this. I may be wrong, but I feel like this issue only started happening this month because I did not notice this behavior in an application I was working on around a month ago.