Pressing up and getting the last chat message into the chat box again is a base game feature, isn't it?
I sometimes got the problem, that after login, my first post of a message in a chat channels is not written to chat. If I repeat it (with the up key or manually) it's in the chat all of sudden (at least only once and not twice then).
If a chat message is not shown a 2nd time after using the up key it shluld be some addon of yours which is "filtering" the last send messages somehow, but EVENT_CHAT_MESSAGE_CHANNEL ist not filtering! You cannot suppress chat messages with this event afaik.
Search your addons for
Code:
CHAT_SYSTEM.OnChatEvent
, normally the filtering is applied in there.