diff --git a/interview_cake/math/find_dup.py b/interview_cake/math/find_dup.py
new file mode 100644
index 0000000..7d2303f
--- /dev/null
+++ b/interview_cake/math/find_dup.py
@@ -0,0 +1,38 @@
+#!/bin/python
+
+"""
+Find a duplicate
+We have a list of integers, where:
+The integers are in the range 1..n1..n
+The list has a length of n+1n+1
+It follows that our list has at least one integer which appears at least twice. But it may have several duplicates, and each duplicate may appear more than twice.
+
+Write a function which finds an integer that appears more than once in our list. (If there are multiple duplicates, you only need to find one of them.)
+"""
+
+def find_dups(num_list):
+    num_dict = {}
+    for n in num_list:
+        if n in num_dict.keys():
+            num_dict[n] += 1
+        else:
+            num_dict[n] = 1
+    
+    for k,v in num_dict.items():
+        if v > 1:
+            print "dup is {}".format(k)
+
+def find_dups_set(num_list):
+    
+    num_set = set()
+
+    for n in num_list:
+        if n in num_set:
+            print n
+        else:
+            num_set.add(n)
+
+
+num_list = [6,1,3,7,6,4,5,2,8,5,6,6,7]
+find_dups(num_list)
+find_dups_set(num_list)