The double chin is of course not actually a double chin in the sense that you have two chins. When someone refers to you having a double chin it is most likely because you have a roll of fat which is coming up behind your chin and making it appear as if you have a kind of double chin. To dream of having two chins would be something entirely different.
If you have a dream about a double chin then this means that you feel like you are getting fat in real life. The double chin is known as a symbol of unattractiveness and of fatness. Even if you are not actually overweight yet in real life you should still consider why you had this dream. You do not dream that you are getting fat for no reason, this is just a fact, so if you have a dream in which you are getting a double chin it could mean that you have been making some unhealthy decisions in the waking world that are leading to you feeling like this. Try not to be so unhealthy and then things should work themselves out for you if this is the case.
If you have a dream that you have a double chin this could also mean that you've been greedy lately. Being overweight is a sign of greed and lust and the inability to control your urges and desires so if you have a dream about a double chin it usually means you've been too greedy in some situation recently. The best way to handle this is by amending your ways and making sure that everyone has a say in a situation. And never take more than your share of anything.