I am making a moth for use in Unity and it is flying great, but I need to double the png file so it is the same front and back. Is it possible to duplicate within the UV editor or do I have to make a doubled image in gimp and hope it fits?
I don't think you can duplicate the image within the blender UV editor. However, you can as you said create another image using GIMP. In the UV editor go to UVs > Export UV layout to have an image of your UVs. Then you import it in GIMP and you can edit it. That way, you can be sure it will fit ! You can also make your 2 UV islands overlap in the UV map of your moth. It is pretty quick and you just have to reimport your model in Unity, but I don't think it is something to do when you are making a game (something to do with lightmaps or something like this....). I am not sure I am being clear enough ...
Hope it will help !
In fact you can transform your object to a double-sided material (under Blender Render: "Object data properties">Texture Face options>Two-side; under Blender Cycles: Geometry Node>Backfacing> connect it to a mix shader fac that one of the shader is one image and the other shader is the other image), and use that same image for the other side (if fact you can even use another image for the other side - that's how people make magazines and books in blender), or if you want/need a mirrored image, you can (to the other face), re UVmap it, press S > press X > press -1, to mirror the UVmap under X-axies and the image will render mirrored. To prevent strange artifacts, you might want to dupli your object and call it "back side", flip the normals, UV map it and mirror the UV-map..