One off payment keeps being paid in SelectPay

The one off payment pay record should have a Fixed End Date added to it, matching the effective date of the pay element to ensure that the rate is set to 0.00 in SelectPay for the following period after the payment.
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SelectHR will not exclude one off payments from the pay run in SelectPay. It will just set the rate to 0.00, so you will need to exclude the pay element in SelectPay manually, if you wish to do so.