#!/usr/bin/python

user_names = []
with open(filename) as f: # change the filename to "yourfile.txt"
    for l in f:
        if "@" in l:
            # it seems we can assume every line we're interested in tags a user
            pieces = l.split()
            user_names.append(pieces[0]) # first word on the line is a username
            user_names.extend([p for p in pieces[1:] if p[0] == '@']) # find everything else that starts with @

updated_user_names = [u if u[0] == '@' else '@' + u for u in user_names]

with open("output.txt", "w") as f:
    for u in set(updated_user_names):
        f.write(u + "\n")