check if it is running when calling start(), avoid multiple run() goroutine

cron.go

@@ -123,6 +123,9 @@ func (c *Cron) Entries() []*Entry {
 
 // Start the cron scheduler in its own go-routine.
 func (c *Cron) Start() {
+	if c.running {
+		return
+	}
 	c.running = true
 	go c.run()
 }