How can I execute an insert statement when duplicate field is found in MYSQL?

I am trying to create a mySQL statement that will insert a field when a duplicate is found for each duplicate.

  ID | IDcode | Size 
   1    1        S
   2    1        M
   3    2        L
   4    3        S
   5    4        M

For Example when IDcode is duplicate i need to insert a statement for size S and Size M.