Two ways to clone someone in a picture using GIMP
I highly recommend getting yourself some high quality free software like GIMP to do your image manipulation. Now, how do I make a twin of myself in a picture using GIMP? There are actually two ways to do this.
Use the eraser and layers to make a clone
One method is to use two layers, one placed on top of the other one, and then you use the eraser to erase the top layer. As the guy in the tutorial mentioned, this method makes a change to the data in the actual photograph, so if you make a mistake, there’s no going back.
Use a layer mask
Method number two is using a layer mask in your photo manipulation to create a mask out of one layer, allowing the bottom layer to show through. If you make a mistake this way, you can always go back and fix your layer mask as if it was its own layer. Be sure to choose “White (Full opacity)” when selecting your mask choice.
Learn more about image manipulation with GIMP.