Do not send backoffice receipts for contributions that are pending
We have found on a client site as per https://github.com/iATSPayments/com.iatspayments.civicrm/issues/225 that if you use an ACH / EFT as Iats does or the payment is completed via notification which might be the case for some processors you can wind up with 2 receipts being issued. One when the back office form is completed and the other when the payment is actually successful.
If the send receipt checkbox is checked on the back office contribution and the contribution remains pending a receipt is issued
Only one Receipt should be issued when the contribution is completed