You can quickly copy a shape in Adobe Illustrator by down the Option key (for a Mac) or the Alt key (for a PC) then clicking and dragging on the shape you want to copy. When you release the mouse, the copy of the shape will be put where the mouse is, and the original shape will be left in shape. Yeah, you could copy the shape and paste it (which is useful if you are pasting it multiple times), but this method is a lot quicker and it gives you better control over the placement of the pasted shape.