diff --git a/scripts/moby.sh b/scripts/moby.sh
index f2196ff18..31534ca7f 100755
--- a/scripts/moby.sh
+++ b/scripts/moby.sh
@@ -2,15 +2,24 @@
 
 set -e
 
+mobydir=/Applications/Docker.app/Contents/Resources/moby
+
+backup_once() {
+	if ! [ -e "$1"- ]
+	then
+		cp "$1" "$1"-
+	fi
+}
+
 if [ "$1" = "undo" ]
 then
-	cp /Applications/Docker.app/Contents/Resources/moby/initrd.img- /Applications/Docker.app/Contents/Resources/moby/initrd.img
-	cp /Applications/Docker.app/Contents/Resources/moby/vmlinuz64- /Applications/Docker.app/Contents/Resources/moby/vmlinuz64
+	cp "$mobydir"/initrd.img- "$mobydir"/initrd.img
+	cp "$mobydir"/vmlinuz64- "$mobydir"/vmlinuz64
 else
-	cp /Applications/Docker.app/Contents/Resources/moby/initrd.img /Applications/Docker.app/Contents/Resources/moby/initrd.img-
-	cp /Applications/Docker.app/Contents/Resources/moby/vmlinuz64 /Applications/Docker.app/Contents/Resources/moby/vmlinuz64-
-	cp alpine/initrd.img /Applications/Docker.app/Contents/Resources/moby/initrd.img
-	cp alpine/kernel/x86_64/vmlinuz64 /Applications/Docker.app/Contents/Resources/moby/vmlinuz64
+	backup_once "$mobydir"/initrd.img
+	backup_once "$mobydir"/vmlinuz64
+	cp alpine/initrd.img "$mobydir"/initrd.img
+	cp alpine/kernel/x86_64/vmlinuz64 "$mobydir"/vmlinuz64
 fi
 
 killall com.docker.driver.amd64-linux