One record represents the first installment, the other record represents the schedule of payment. Think of the gift of the type "Recurring gift" as the folder and the "Recurring gift payment" as files inside that folder.
When looking at the records created when a recurring plan's first installment is synced to Raiser's Edge NXT, you will see both a Recurring gift record and a Recurring gift payment record. This is intentional, as future installments will only create a Recurring gift payment record also associated to that Recurring gift record.
Recurring gift - This is the record the houses all the payments that belong to the recurring plan. Each recurring plan synced from Fundraise Up will have a Recurring gift record in NXT. Cancel the plan and start one for the same donor? You will see a new Recurring gift that belongs to that donor.
Recurring gift payment - This refers to each payment installment that belongs to that plan. Payment installments are charged on a set frequency set by the donor. Each installment creates a new Recurring gift payment record associated with the Recurring gift record that represents the plan.
For more information on how to customize the Raiser's Edge NXT Integration please visit the article on Customizing the Blackbaud Raiser's Edge NXT Integration