I recently bought a house and have a slight leak under my kitchen sink. Was trying to figure out where it was coming from this morning so I could fix it, when I found something that makes no sense to me.
There is a filtered water dispenser on sink, on the right 
which has two drainage lines coming out of it underneath 
that feed into the drainage pipe from the dishwasher 
I'm really confused by this. I don't understand why it would need one drainage line, let alone two. What am I missing here?
Best Answer
I found a good explanation at: http://www.purewateroccasional.net/hwairgapfaucet.html
The filter (possibly Reverse Osmosis) part has a drain pipe. But it doesn't go directly to the drain. Goes up to the tap base, where it dribbles in the open air (the "air gap") and back down through the other pipe to drain away.
If the second drain line becomes blocked, this "air gap" can leak. Not sure if the possible leak would only occur when the tap is in use.