diff --git a/interfaces/13-reflection-2/main.go b/interfaces/13-reflection-2/main.go
index 9629cc7..70ebdfc 100644
--- a/interfaces/13-reflection-2/main.go
+++ b/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))
diff --git a/interfaces/14-io-reader/main.go b/interfaces/14-io-reader/main.go
index 023bea7..4d8244a 100644
--- a/interfaces/14-io-reader/main.go
+++ b/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))
diff --git a/interfaces/15-io-reader-composition/main.go b/interfaces/15-io-reader-composition/main.go
index 812e47c..35b6412 100644
--- a/interfaces/15-io-reader-composition/main.go
+++ b/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))