fix: Toggle 'fast_unique' on new_from_index

[ritchie46]

fixes #19950

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.46%. Comparing base (8d9ed64) to head (62f1ac3). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #19956 +/- ## ======================================= Coverage 79.46% 79.46% ======================================= Files 1555 1555 Lines 216286 216288 +2 Branches 2456 2456 ======================================= + Hits 171876 171884 +8 + Misses 43852 43846 -6 Partials 558 558 ```

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