When you enable text search debugging, the contextual search service returns duplicate results.

 

1. Navigate to Self-Service > Knowledge.
2. Search for term "email". Observe that there are 22 results and 2 requests made (first window 0-19, second window 20-40) and unique results are displayed.
3. Navigate to System Diagnostics > Session Debug > Debug Text Search.
4. Repeat Steps 1 and 2 and note that the last two results of the first window are returned as duplicates in the second window, while the actual results of the second window are not returned at all:

This is expected behavior.

When debug logging is enabled for text search, the order of displaying results having the same score can change, as affected by the multiple additional queries running.

This is giving the impression of duplicate results, but if you increase the page size for the result output set, you'll see there are no duplicates.