Clarification about sets

I indicate that a set doesn't contain duplicate elements and add a little example

python3.html.markdown

@@ -7,6 +7,7 @@ contributors:
     - ["Zachary Ferguson", "http://github.com/zfergus2"]
     - ["evuez", "http://github.com/evuez"]
     - ["Rommel Martinez", "https://ebzzry.io"]
+    - ["Roberto Fernandez Diaz", "https://github.com/robertofd1995"]
 filename: learnpython3.py
 ---
 
@@ -352,6 +353,8 @@ valid_set = {(1,), 1}
 # Add one more item to the set
 filled_set = some_set
 filled_set.add(5)  # filled_set is now {1, 2, 3, 4, 5}
+# Sets do not have duplicate elements
+filled_set.add(5)  # it remains as before {1, 2, 3, 4, 5}
 
 # Do set intersection with &
 other_set = {3, 4, 5, 6}