Write a piece of code that counts the number of duplicate elements in a linked list, that is, the number of elements whose values are repeated at an earlier index in the list. Assume that all duplicates in the list occur consecutively. For example, the list [ 1, 1, 3, 5, 5, 5, 5, 7, 7, 11] contains five duplicates: one duplicate of element value 1, three duplicates of element value 5, and one duplicate of element value 7.