Database unique still enqueues duplicate items

This happens because, in a single request, we can add the same item more than once. (I think because the same URL was invalidated by multiple tags). A unique constraint would be a better solution but I didn't want to make such big changes to the module in a patch. Instead, I've checked if the item has already been added to $_items before trying to add it again.

Merged. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.