Fun With Faulty Linked Lists

Suppose that you have a faulty, singly-linked list where at some point the list cycles into itself.  For example:

1 -> 3 -> 7 -> 4 -> 2
          ^         |
          |         |
          +---------+

Here, the element 7 has two pointers entering it, one from 3 and one from 2.  In O(N) time (where N is the length of the list), O(1) space, and without modifying the elements in the list, return a pointer to the (unique) element in the list with two incoming edges.