diff --git a/challenges/firstDuplicate/firstDuplicate.py b/challenges/firstDuplicate/firstDuplicate.py
new file mode 100644
index 0000000..1b84a21
--- /dev/null
+++ b/challenges/firstDuplicate/firstDuplicate.py
@@ -0,0 +1,11 @@
+def firstDuplicate(a):
+    seen = set()
+    for num in a:
+        if num in seen:
+            return num
+        seen.add(num)
+    return -1 
+
+assert firstDuplicate([2, 1, 3, 5, 3, 2]) == 3
+assert firstDuplicate([2, 2]) == 2
+assert firstDuplicate([2, 4, 3, 5, 1]) == -1