Double sound notification when plugging in iPhone X with Fast Charge
Yes, when fast charging iPhone gives the charging sound ("Ding") notification twice. And definitely you will not get sound notification twice when you have used the 12W iPad charger.
12W Charger => The downside is that while it’s faster than the brick that comes with your phone, it’s still not quite the true quick charging speed that a USB-C brick should offer. Simply you could not get the true fast charging. But it is better than what comes with the box.

29W Charger => Apple isn’t saying what the maximum fast charging speed is for the new phones — only that it achieved those fast charging results with its 29W, 61W, and 87W USB-C chargers for the new MacBooks and MacBook Pros. These chargers offer the best fast charging power where you hear the charging sound twice.

References : (EasyAcc, 2017), (Apple Inc., 2017) , (The Verge, 2017).