Duplicate Words

Background:

What background? Seemed like a good idea at the time...

Input:

Anyway, you're to write a program that reads in a whole bunch of words (each word is no more than 20 characters) and spit out the duplicate words. For our purposes, words are to be separated by spaces, commas (,), !, ?, and periods (.). No word goes past the end of a line. There can be an arbitrary amount of duplicate words. All comparisons should ignore case.

Output:

Output the duplicate words, one per line, in lower case and in sorted order.

Sample Input:

How much wood would a woodchuck chuck if a woodchuck could chuck wood? Well? what do you think? Are you feeling well?

Sample Output:

a
chuck
well
wood
woodchuck
you