When you draw an image for tracing, you want to make sure to use clean lines (eg: Sharpie Marker rather than pencil) on white paper.
That’s because a cut line will be placed anywhere there is a change of color…from black to white, or white to black. So you want to make sure that any closed shapes are completely closed, with no gaps in the line. That will create a single cut line that loops around on itself, which acts like a double cut line.
The other thing to watch is that you do not click directly on the line itself, if you want to just cut out around it. Click on the white background. Otherwise the software assumes you want to place a cut line on the outside of the line that you clicked on, and one on the inside of the line. There is a color change to white on either side of a closed black line.
There is a tutorial on how to use the Trace tool in the Glowforge interface here: