Or, maybe
prod_id_to_channel_set = defaultdict(set)
prod_id_to_channel_set[prod_id].add(channel)

and then in handle collisions:
collisions = [x for x,y in prod_id_to_channel_set if len(y) > 1]
if not collisions:
    return
print "kaboom, collisions"
for prod_id in collisions:
     print "product id:"
     for channel in sorted(prod_id_to_channel_set[prod_id]):
          print channel

meh, I give up. Whatever works is ok with me.