Yeah, that’s fairly common with new Illustrator users. Click on one of the squares and check to see if it has a Fill color showing, or a Stroke color.
If it shows a Fill color but no Stroke color, remove the fill color and set the Stroke color to black. Then switch to Outline mode, zoom way in, and you will be able to see how many Cut lines are actually there. Delete one of them and you should be good to go.
There’s a tutorial here that explains what is going on in slightly more technical detail:
Sometimes what looks like a single stroked line is actually a filled compound shape. You just need to release the compound path, get rid of all the fill and the interior shape, and assign a Stroke color to the remaining path.