Don't forward duplicate JOIN commands

Closes: https://todo.sr.ht/~emersion/soju/171

downstream.go

@@ -1771,6 +1771,11 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 				continue
 			}
 
+			// Most servers ignore duplicate JOIN messages. We ignore them here
+			// because some clients automatically send JOIN messages in bulk
+			// when reconnecting to the bouncer. We don't want to flood the
+			// upstream connection with these.
+			if !uc.channels.Has(upstreamName) {
 				params := []string{upstreamName}
 				if key != "" {
 					params = append(params, key)
@@ -1779,6 +1784,7 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 					Command: "JOIN",
 					Params:  params,
 				})
+			}
 
 			ch := uc.network.channels.Value(upstreamName)
 			if ch != nil {