Removing a child from an incomplete duplicate parental relationship

 This is the situation where a child is listed twice (with the same ID):

The child is shown once with both parents and once with just one parent, as illustrated below. Note that Henry Crockford is listed twice with the same ID.

If the ID of the "single" parent or the child differs from the ID of the corresponding person in the couple relationship, merge the duplicates.

If the child is still shown additionally with a "single" parent, the easy solution is to click the edit icon to the right of the child shown with only one parent. Then remove the child from that relationship. And the box for the "single" parent should disappear when the screen is refreshed.

Sometimes you can solve the problem by adding the missing parent, but frequently it will bring up an error message saying that the relationship already exists. In my experience, removing the child from the incomplete relationship always works. 

As you help others with family history, you will often encounter this issue. And if you know the process, it can be solved in a couple of minutes.