The incoming message was part of a replay of message history.
In XMPP multi-user chat, a few past messages are replayed when you join a chatroom. A sufficiently capable IRC connection manager could also set this flag on historical messages when connected to a proxy like bip or irssi-proxy. The existence of this flag allows loggers and UIs to use better heuristics when eliminating duplicates (a simple implementation made possible by this flag would be to avoid logging scrollback at all).