That isn't supposed to happen.  An image is assigned to a page, and a page belongs to exactly one document.  Are you sure that's what you're seeing?  You aren't looking at a shortcut?

You should open a support case about this.  If the image really is assigned to two documents, you might run into trouble when you delete one of the documents or move it to another volume.  Support can also help you determine if there are other instances of this problem.