This list seems to work with ONE tag only !
maybe i need another extension specific

Thanks a lot beacuse my website is almost finished and I import all my item with many tags !

Are you sure your items have these tags in common?

If you imported your items and tags correctly then you should clear your cache as well.

PS. This attitude is not helpful at all

Yes I'm sure and check it before !

I look at the files item.php and itemlist.php and it seems the request doesn't make a "count" and just a pass
. It uses the attribute DISTINCT :

$query = "SELECT DISTINCT itemID FROM #__k2_tags_xref WHERE tagID IN ({$sql}) AND itemID!={$itemID}";

Look here
getk2.org/community/advsearch?q=getRelatedItems&exactname=1&searchdate=all&childforums=1&limit=5

I should try this soluce I think but i don't look for a specific way of "overload"

Thanks

You can try this solution which seems to fit your needs and see it works for version 2.6.x ,BUT you have to make sure you backup first and always keep your edited files backed up because you will lose these changes every time you update K2.

yes for sure

Is there a specific way to overlaod K2 ? I don't look any specific directory or documentation about this

Thanks

If you mean override, yes you can override all of its templates and modules.
This tutorial will help you. getk2.org/documentation/tutorials

You can always develop plugins as well for the core functionality, but the solution you found requires core hacking.

Thanks for the tip.

Just a question about this. Will the developpers add this functionnality in a next release?
If not. Is there a way to find somebody who would write the plugin?

Any suggestions ;)