Sounds well beyond trouble shooting. You'll need to restore one of them from a recent backup. If it isn't the role holder then you can seize roles.
https://support.microsoft.com/en-us/help/255504/using-ntdsutil-exe-to-transfer-or-seize-fsmo-roles-to-a-domain-control
then perform cleanup.
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/ad-ds-metadata-cleanup
Then build a new replacement for the other one. It's always recommended to have at least two active domain controllers to maintain high availability and for disaster mitigation.
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