lnwallet/chancloser: ignore spurious channel flushed events

If we go to close while the channel is already flushed, we might get an extra event, so we can safely ignore it and do a self state transition.

lnwallet/chancloser/rbf_coop_transitions.go

@@ -582,6 +582,12 @@ func (c *ClosingNegotiation) ProcessEvent(event ProtocolEvent, env *Environment,
 	// we receive a confirmation event, or we receive a signal to restart
 	// the co-op close process.
 	switch msg := event.(type) {
+	// Ignore any potential duplicate channel flushed events.
+	case *ChannelFlushed:
+		return &CloseStateTransition{
+			NextState: c,
+		}, nil
+
 	// If we get a confirmation, then the spend request we issued when we
 	// were leaving the ChannelFlushing state has been confirmed.  We'll
 	// now transition to the StateFin state.