Because they are two events. The invoice records the sale and what the customer owes you. The receipt records the money arriving. The income is counted only once.
How each one posts
- Invoice: the customer's balance goes up in Accounts receivable, and the sale goes to Invoice sales plus VAT payable.
- Receipt: the money goes into Cash on hand or Bank & cards, and Accounts receivable goes down by the same amount.
The receipt never touches income, so Profit & Loss shows the sale once.
Good to know Open the customer in Customer Ledger to see both lines side by side in their Statement of account, with what is still owed.