Seems as if the addon ist using ht normal debug output function d() ( d(lootMessage) ) which writes the message to the system chat.
And in additon it uses another window for the addon itsself, named GroupLootWindowBuffer?
if GLSettings
isplayInChat() then d(lootMessage) end
GroupLootWindowBuffer:AddMessage(lootMessage, 255, 255, 0, 1)
Maybe you did activate the system messages in ALL your chat tabs?
Right click the chat tabs, choose settings, and check if the SYSTEM entry is marked for all chat tabs.
Remove the SYSTEM checkbox for all but 1 tab then.