Write a java method called "keepDistinct" that receives the head of a doubly linked list and removes nodes containing duplicate values (i.e. if two or more nodes have the same value only one one of them will remain the rest must be eliminated.

Important notes:

1) Assume that the linked list contains integers.

2) You may write helper methods if needed.

3) Do not use recursion.

4) You are not allowed to use another data structure (like array, set, ...) to store the list in it terporarily.