diff --git a/common_ds_algo_problems/Find Duplicate b/common_ds_algo_problems/Find Duplicate
new file mode 100644
index 0000000..d15b27e
--- /dev/null
+++ b/common_ds_algo_problems/Find Duplicate	
@@ -0,0 +1,36 @@
+#include <iostream>
+using namespace std;
+int duplicateNumber(int *arr, int size)
+{
+    int i;
+    int newsum=((size-2)*(size-1))/2;   //n(n+1)/2 sum of n natural no.
+    int sum=0;
+    for(i=0;i<size;i++){
+    sum=sum+arr[i];
+    }
+    return sum-newsum;
+} 
+
+
+int main()
+{
+
+	int t;
+	cin >> t;
+	
+	while (t--)
+	{
+		int size;
+		cin >> size;
+		int *input = new int[size];
+
+		for (int i = 0; i < size; i++)
+		{
+			cin >> input[i];
+		}
+
+		cout << duplicateNumber(input, size) << endl;
+	}
+
+	return 0;
+}