Please note that the check should work the other way round as well, i.e. disallow setting a mul label such that it, combined with the present item’s English description, conflicts with another item’s English label and description. Or another item’s mul label and English description. And of course the same for any other language as well. And this is where things may become tricky performance-wise (although I don’t know how this check is implemented, so I can’t tell for sure): if one changes the mul label of an item with n descriptions, the software needs to check 2n label–description pairs across all items in Wikidata, which is a huge increase from the current one item–description pair. I hope it will be possible to implement this without bringing down Wikidata (especially as one of the reasons for mul was to avoid bringing down Wikidata).

I agree a check should be implemented to avoid duplicates.

Actually, the user would likely first create an item, then add the native label, then add a description, then add statements and move on to the next item. When the check is enabled, it's likely the user just sees he can't use this particular description and writes another description, or in automated editing, ignores this error. So not particularly effective overall, only QuickStatements can benefit from it since it creates the item in full in one edit.

Conflicts within one language (e.g. English label/description pair of one item conflicting with the English label/description pair of another) have long been rejected, so the rejection is probably at least somewhat effective (otherwise someone should’ve disabled the check). I personally, if I get such an error message, look up the other item to see if they’re duplicates and could be merged. This includes when I create name items, and I hope other people also do the same.