Hi have paid my invoice twice and responded to an email to receive a credit. I have still not received a credit - what do i do?

If this is your own Salesforce subscription invoice, the community can't see your account, so it is best handled directly by Salesforce Billing. Open a billing case from the Salesforce Help portal (Help > Contact Support, choose a billing or invoice topic). Attach both payment confirmations and the email thread where the credit was offered, and ask whether it will be applied as a credit memo against the next invoice or refunded. 

If this was an invoice from another company that uses Salesforce Billing, reply to that company's accounts team with both payment references. They issue the credit note in their own org.