If you draw a rectangle divided up into eight even parts, and label them each as 1/8, then you can show your work for how you double it. You do this by "coloring" one in, and then doing the same for a second piece (but color that one in a different way). You can visually double an eighth in that way.
Or, you can simply multiply by 2 to double 1/8:
1/8x2/1=2/8