Where can I get a duplicate of the marriage certificate?

The reissuance of a certificate of state registration of a civil status act, the original of which was stolen, lost, damaged or destroyed, is carried out by departments of state registration of civil status acts, diplomatic missions and consular institutions of Ukraine on the basis of the act record of civil status at the request of the person for whom the record was made, parents, adoptive parents, guardians, custodians, a representative of a health care institution, an educational or other children's institution where the child is permanently staying, a guardianship and guardianship authority.

If you got married on the territory of the former USSR, you can get a duplicate only on the territory of the former union republic (Russia, Kazakhstan, Belarus, etc.).