The GP Support team explains reasons behind Benefit, Deduction and/or Pay Code Amounts are Doubled on My W-2s
Benefit, Deduction and/or Pay Code Amounts are Doubled on My W-2s – Dynamics GP Support and Services Blog – Microsoft Dynamics GP – Microsoft Dynamics Community
Payroll & HRMicrosoft & the GP EcosystemDevelopment & Integration
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.