This should not be a problem going forward for new questions, since recent changes to the closing system look out for this.
However, there are some questions still in the database and stuck in this state - where the duplicate named by a question that was marked as a duplicate has been deleted independently of the question pointing to it.
If you see this, just flag it for moderator attention as 'other' and let them know that you've found a dead end. If the question itself seems reasonably good, it may be able to just be re-opened, or pointed at a better canonical question that has plenty of good answers. If you have a suggestion either way, it would be very helpful to include that in your flag.
Other times, especially if it's just remnants of a user asking the same question repeatedly, a speedy deletion is probably in order.