Michael

>> exists($vSeq[index-of($vSeq,.)[2]][1] )
>
> I think that if there are no duplicates, this is O(n^2), whereas the
distinct-values solution is O(n log n). Harder to judge how they compare if
duplicates are more probable: I think this is O(m*n) where n is the size of
the sequence and m is the expected number of items between two duplicates,
i.e. m=1/p where p is the probability of an item being a duplicate.

I agree the above is O(n^2) with no duplicates. However I wonder how
distinct-values is implemented with Saxon that you say it is O(n log n). What
intrigues me is that the order does not change on Oxygen 16.1.

Cheers
Leo