diff --git a/List7.scala b/List7.scala
new file mode 100644
index 0000000..59e67ab
--- /dev/null
+++ b/List7.scala
@@ -0,0 +1,24 @@
+object List7 extends App {
+  def duplicate(list: LazyList[Int], reps: LazyList[Int]): LazyList[Int] = {
+    def containsDuplicate (list: LazyList[Int]): Boolean =
+      list match {
+        case LazyList() => false
+        case h #:: t => if (t.contains(h)) true else containsDuplicate(t)
+      }
+
+    def helper(l: LazyList[Int], r: LazyList[Int]): LazyList[Int] =
+      (l, r) match {
+        case (_, LazyList()) => LazyList()
+        case (LazyList(), _) => LazyList()
+        case (_ #:: t1, 0 #:: t2) => helper(t1, t2)
+        case (h1 #:: _, h2 #:: t2) => h1 #:: helper(l, (h2-1)#::t2)
+      }
+    if (containsDuplicate(list)) throw new Exception("List contains duplicates")
+    helper(list, reps)
+  }
+
+  println(duplicate(LazyList(1,2,3),LazyList(0,3,1,4)).force)
+  println(duplicate(LazyList(1,2,3),LazyList(0,3)).force)
+  println(duplicate(LazyList(1,2,3),LazyList(1,4)).force)
+  println(duplicate(LazyList(1,2,1,3),LazyList(0,3,1,4)).force)
+}