The 'double' I think you are referencing is in Germanic lore, called the Doppleganger. It is said that this is either a physical person that is exactly as you are, though a separate body and mind, and that if you would meet, both would die. In modern Occult lore, a Doppleganger is considered a double of your spirit (and, you obviously don't die if you meet). Some people beleive they are real, some don't. It's all highly diverse when it comes to personal spirituality.
If you do beleive in them, communication with them would be the same as talking with any other spirit. You just have to remember that spirits don't always 'talk' in fully verbal sentences, but you can hear them, see them, feel them, smell them (even taste, as smell is it's mother sense), simply know what they are trying to say. All of this, however, comes with a gut feeling. Instinct is your best friend when it comes to anything spiritual, along with reason.