Graham
I think what aghadowey says is spot on. There are quite a few examples of this in our local cemetery. It usually happens when a child's death pre-dates that of the parents and is memorialised on the parent's grave as well as the original grave.
The attached photo is an example of this, with the added problems that the memorial was incomplete and was in the totally wrong place. I've attached two PDFs; the first shows the full text of the two headstones and the second shows the Burial record of the child in question (third from the bottom).
PatB.