Make icon with GIMP - Duplicate the first layer and resize for icon sizes

make-icon.scm

(script-fu-register
	"script-fu-make-icon"                               ;func name
	"Make icon"                                          ;menu label
	"Dupliate the first layer and resize for icon sizes" ;description
	"Mazin SW"                             ;author
	"copyright 2015, Mazin SW"        ;copyright notice
	"Setember 14, 2015"                          ;date created
	""                     ;image type that the script works on
	SF-IMAGE "Image" 0   ;a image variable
	SF-DRAWABLE "Drawable" 0 ;a layer variable
)
(script-fu-menu-register "script-fu-make-icon" "<Image>/Layer")
(define (script-fu-make-icon img drawable)
	(let* (
			(layer 0)
		  )
		(gimp-image-undo-group-start img)

		(set! layer (car (gimp-layer-copy drawable TRUE)))
		(gimp-image-add-layer img layer 0)
		(gimp-layer-scale layer 128 128 FALSE)

		(set! layer (car (gimp-layer-copy drawable TRUE)))
		(gimp-image-add-layer img layer 0)
		(gimp-layer-scale layer 64 64 FALSE)

		(set! layer (car (gimp-layer-copy drawable TRUE)))
		(gimp-image-add-layer img layer 0)
		(gimp-layer-scale layer 48 48 FALSE)

		(set! layer (car (gimp-layer-copy drawable TRUE)))
		(gimp-image-add-layer img layer 0)
		(gimp-layer-scale layer 32 32 FALSE)

		(set! layer (car (gimp-layer-copy drawable TRUE)))
		(gimp-image-add-layer img layer 0)
		(gimp-layer-scale layer 24 24 FALSE)

		(set! layer (car (gimp-layer-copy drawable TRUE)))
		(gimp-image-add-layer img layer 0)
		(gimp-layer-scale layer 16 16 FALSE)

		(gimp-image-undo-group-end img)
		(gimp-displays-flush)
	)
)

Create the file and put on folder %userprofile%\.gimp-2.6\scripts
Open an image with GIMP, click on menu Layer, Make icon