>    I find that very often I end up with lists of elements like:
> 
> 
>   Which I need to trim, keeping only the first element with an unique
> attibute value, or unique attribute values. 
> 
> Is there a clean way to do it ?
> 

Eliminating duplicates is essentially the same problem as grouping: see

http://www.jenitennison.com/xslt/grouping

Michael Kay