1. Tip Effects
1.4 Double/multiple tips
if the tip is broken, or contaminated, you can often get a "double tip" effect, where both the tip and the contamination are scanning the surface.
See the example below.
In the example above, each DNA molecule has a "twin", caused by the double tip.
How to avoid it
You can check that this is occurring with a tip characterisation sample. If it happens, change the tip. Once a tip is broken, you must change it. If it is contamination, you MIGHT be able to clean it. But most often, even in this case, you will have to change to a new tip.