Express one's displeasure, repeating error (3-3)
I believe the answer is:
boo-boo
'error' is the definition.
(I know that boo-boo is a type of error)
'express one's displeasure repeating' is the wordplay.
'express one's displeasure' becomes 'boo' (I can't explain this - if you can you should believe this answer much more).
'repeating' shows that letters should be duplicated.
'boo' duplicated is 'BOO-BOO'.
Can you help me to learn more?
(Other definitions for boo-boo that I've seen before include "Mistake" , "Clanger" .)