The arrival of iOS 11 and iPhone X came with a major change in the design and performance of the App Store. The Apple application store for iOS has undergone a practically complete redesign, in which developer stories and application recommendations are now the highlights. And there’s more: with the arrival of the iPhone X, the way we download the applications has also changed.
The absence of the Home button and the integrated Touch ID sensor are new features of the iPhone X. Now we have to use the facial recognition of the iPhone X to identify us. The so-called Face ID that is always active is in charge of recognizing our face and allowing us to purchase the application that we have requested to download.
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But how do we confirm the purchase to avoid unwanted downloads? Until now, whenever our iPhone had Touch ID, when we downloaded an application for the first time, we had to confirm our identity and our intention to do so by placing our finger on the fingerprint sensor that is integrated just below the home button. This way the iPhone knew that we were the owner of the phone, and proceeded to the purchase and download of the app without having to enter the password of our account. But with the iPhone X, there is no Home button.
The facial recognition is now responsible for the identity confirmation, something that happens automatically just by looking at the screen, which is pretty usual when we are downloading an application.
How to Fix Double Click to install/Pay message?
Well, Apple has added a step that until now did not exist in any of the devices: you have to press the side button twice. 
Double-Click the Side Button This simple gesture makes the system know that we really want to buy an application and that it is not an error, and the download will start automatically. Apple could have opted for a virtual button on the screen, but it was decided that a physical button would work better. The same goes for integrated purchases.
