In Touch with Dynamics (GP) provides a solution for when Checkbook [checkbook id] is already in user by user [username].
In Touch with Dynamics (GP): Dynamics GP Bank Management – Checkbook [checkbook id] is already in user by user [username].
Install, Upgrade & AdminPayables & Receivables
Archived post from 2011. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.