Some Prepayment Invoice amount showing double

Summary:

Content (required):

Dear All

We have an query which will fetch the Prepayment Invoice details , the query is working fine , but it is noticed that some of the invoice amount is showing as double can any one help what could be the problem , code is attached

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):

SELECT 

 '1' source,

         ai.invoice_num,

         ai.invoice_id,

         ai.invoice_date invoice_date,

         prepay.payment_currency_code,

 sum(DECODE(pay.PAYMENT_CURRENCY_CODE,'SAR',pay.amount,pay.PAYMENT_BASE_AMOUNT)) as invoice_amount,