I have some information about why your sales were double. An invoice is used when your customer agrees to pay you later. A sales receipt is used when your customer pays you on the spot for goods or services.
In your case, you'll want to delete one of these transactions to correct your sales entry. Here's how to delete:
Go to the Sales menu. Then, select the All Sales tab.
Tick the Filter drop-down and change the Type to Invoices or Sales Receipt depending on what you want to delete.
Hit Apply.
Locate and double-click the transaction to open. Scroll down and tap More.