fix #results in examples, added test file for keys_only

snippets/has_duplicates.md

@@ -11,6 +11,7 @@ def has_duplicates(lst):
 
 ``` python
 x = [1,2,3,4,5,5]
-has_duplicates(x) # result
-True
+y = [1,2,3,4,5]
+has_duplicates(x) # True
+has_duplicates(y) # False
 ```