Excess Refund claimed by mistakenly

We are exporter

For month of August 2017 we by mistakely we taken the amount of (with payment of Tax & without payment of Tax) in our Gst refund amount.

But department refund both the amount.

Now we want to surrender the excess amount in our gst refund August 2018 but the gst Department said you have to pay the excess amount with interest but in this case we both us and department make the mistake.

As per this we are able to pay the excess amount not interest amount.

Please advice us