#!/usr/bin/env python
import rospy
from std_msgs.msg import Int32

n = 0

def cb(message):
        //rospy.loginfo(message.data*2)
        global n
        n = message.data*2

if __name__ == '__main__':
        rospy.init_node('twice')
        sub = rospy.Subscriber('count_up', Int32, cb)
        //rospy.spin()
        pub = rospy.Publisher('twice', Int32, queue_size=1)
        rate = rospy.Rate(10)
        while not rospy.is_shutdown():
                pub.publish(n)
                rate.sleep()

Create twice.py