CHAT_MSG_CHANNEL and 7 iterations?

I'm writing a simple chat filter and am having a doozy of a time trying to track down a problem.

The last line in my lua file is:
ChatFrame_AddMessageEventFilter("CHAT_MSG_CHANNEL", filter)
which should just call the filter every time there's a new message in a followed channel.

The first line of my filter is:
DEFAULT_CHAT_FRAME:AddMessage(msg);
for debug purposes.

My problem is that the message is repeated in the chat window 7 times. I've disabled all of my other addons hoping there was some sort of interaction. Doing a "/chatlist" from the chat window only comes up with only 4 channels. Is there some obvious thing I'm doing wrong? Thanks.