Hi Elizabeth. Do you mean that dragging the selection makes a copy? It doesn't sound exactly like that, but if it is, then are you holding down Alt (Opt) while dragging? Does the twin image persist after you drag it, or become just the one after you release the mouse button?
If it is a display glitch then it might be a video card driver issue.
What operating system are you using?
Would a screen shot help show us what you are seeing? You should be able to paste a screen shot directly to you post.