Why after I purchased, it deducted double amount on my account? I purchased 8$ and it deducted 16$ and this happened many time.

Thanks for posting in the Apple Support Communities ShahrulGhulam,

We understand you're seeing charges for more than you expect, and we'll be happy to assist.

Using the steps on the following page, you can find an itemized view of charges made through your Apple ID, so you can determine what has been charged: See your purchase history for the App Store, iTunes Store, and more

You can also find additional guidance to help investigate unknown charges here: If you see 'apple.com/bill' or an unfamiliar charge on your statement

We hope this helps!