I copied a small clip and moved it to the location in the pic below.
 
When I tried to lengthen it I found a 2nd clip behind it. When I tried to separate the two I couldn't. The 3rd frame shows where they can be selected and resized separately but they won't come apart, like they are locked together somehow.
 
After some tinkering I was finally able to get them apart and delete the rouge double.
 
You don't know this is happening unless you zoom in on the clip or try to edit it.
 
What's causing it to happen?
 

What I've had to do is scroll back one of the clips and click on the 2nd one underneath.  Then you can move it separately.

@

I can't remember how I got them apart but selecting the bottom clip could have been it.

I'll try that the next time it happens.
 
Steve