Duplicate SSN

The U.S. Social Security Administration has made a terrible mistake. When assigning new Social Security numbers (SSN) to U.S. citizens they accidently assigned some duplicate numbers. Fortunately, they caught this mistake early and have a list all the possible duplicates. The Social Security Administration must contact all the people that do not have a unique SSN. Your job is to read the list of Social Security numbers and find all the duplicates. You will then print the list of duplicates in ascending order.

The input is a list of Social Security numbers, one per line. Input will be terminated with the Social Security number 000-00-0000, which is not a duplicate.

A list of all Social Security numbers, in ascending order, that appear more than once as input.