That sketch has lines stacked on top of one another. Below is what I got when I right-clicked on one of them and chose the "Select Other" icon. As you can see, there are two lines there, and it happened on all four lines. Delete one in each place and your extrude will work fine.
I don't know how it happened. I just know they were there. I've never heard of lines spontaneously generating without user input, but I suppose anything is possible.
That sketch has lines stacked on top of one another. Below is what I got when I right-clicked on one of them and chose the "Select Other" icon. As you can see, there are two lines there, and it happened on all four lines. Delete one in each place and your extrude will work fine.