fix: err in interfaces

interfaces/13-reflection-2/main.go

@@ -24,7 +24,6 @@ func main() {
 
 	db := database{list: &store}
 	db.register("book", new(book))
-	db.register("book", new(book))
 	db.register("game", new(game))
 	db.register("puzzle", new(puzzle))
 	db.register("toy", new(toy))

interfaces/14-io-reader/main.go

@@ -41,7 +41,6 @@ func main() {
 
 	db := database{list: &store}
 	db.register("book", new(book))
-	db.register("book", new(book))
 	db.register("game", new(game))
 	db.register("puzzle", new(puzzle))
 	db.register("toy", new(toy))

interfaces/15-io-reader-composition/main.go

@@ -42,7 +42,6 @@ func main() {
 
 	db := database{list: &store}
 	db.register("book", new(book))
-	db.register("book", new(book))
 	db.register("game", new(game))
 	db.register("puzzle", new(puzzle))
 	db.register("toy", new(toy))