diff --git a/apps/services/auth/admin-api/src/app/v2/delegations/test/delegation-admin.spec.ts b/apps/services/auth/admin-api/src/app/v2/delegations/test/delegation-admin.spec.ts
index 19cc29c1c92f..6d0637535db9 100644
--- a/apps/services/auth/admin-api/src/app/v2/delegations/test/delegation-admin.spec.ts
+++ b/apps/services/auth/admin-api/src/app/v2/delegations/test/delegation-admin.spec.ts
@@ -25,6 +25,7 @@ import { getModelToken } from '@nestjs/sequelize'
 import { faker } from '@island.is/shared/mocking'
 import { TicketStatus, ZendeskService } from '@island.is/clients/zendesk'
 import { NationalRegistryClientService } from '@island.is/clients/national-registry-v2'
+import { ErrorCodes } from '@island.is/shared/utils'
 
 const currentUser = createCurrentUser({
   scope: [DelegationAdminScopes.read, DelegationAdminScopes.admin],
@@ -339,6 +340,85 @@ describe('DelegationAdmin - With authentication', () => {
       expect(res.status).toEqual(400)
     })
 
+    it('POST /delegation-admin should not create delegation since it already exists', async () => {
+      // Arrange
+      const { toNationalId, fromNationalId } = {
+        toNationalId: '0101302399',
+        fromNationalId: '0101307789',
+      }
+
+      mockZendeskService(toNationalId, fromNationalId)
+
+      const existingDelegation = await factory.createCustomDelegation({
+        fromNationalId,
+        toNationalId,
+        domainName: null,
+        scopes: [{ scopeName: 's1' }],
+        referenceId: 'ref1',
+      })
+
+      const delegation: CreatePaperDelegationDto = {
+        toNationalId: existingDelegation.toNationalId,
+        fromNationalId: existingDelegation.fromNationalId,
+        referenceId: 'ref2',
+      }
+
+      // Act
+      const res = await getRequestMethod(
+        server,
+        'POST',
+      )('/delegation-admin').send(delegation)
+
+      // Assert
+      expect(res.status).toEqual(400)
+      expect(res.body).toMatchObject({
+        status: 400,
+        type: 'https://httpstatuses.org/400',
+        title: ErrorCodes.COULD_NOT_CREATE_DELEGATION,
+        detail: 'Could not create delegation',
+      })
+    })
+
+    it('POST /delegation-admin should not create delegation since the delegation id already exists', async () => {
+      // Arrange
+      const { toNationalId, fromNationalId } = {
+        toNationalId: '0101302399',
+        fromNationalId: '0101307789',
+      }
+
+      mockZendeskService(fromNationalId, toNationalId)
+
+      const existingDelegation = await factory.createCustomDelegation({
+        fromNationalId,
+        toNationalId,
+        domainName: null,
+        scopes: [{ scopeName: 's1' }],
+        referenceId: 'ref1',
+      })
+
+      // Send in opposite national ids so they will not exist in db
+      const delegation: CreatePaperDelegationDto = {
+        toNationalId: existingDelegation.fromNationalId,
+        fromNationalId: existingDelegation.toNationalId,
+        referenceId: 'ref1',
+      }
+
+      // Act
+      const res = await getRequestMethod(
+        server,
+        'POST',
+      )('/delegation-admin').send(delegation)
+
+      // Assert
+      expect(res.status).toEqual(400)
+      expect(res.body).toMatchObject({
+        status: 400,
+        type: 'https://httpstatuses.org/400',
+        title: ErrorCodes.REFERENCE_ID_ALREADY_EXISTS,
+        detail: 'Delegation with the same reference id already exists',
+      })
+    })
+
     it('POST /delegation-admin should not create delegation with incorrect zendesk ticket status', async () => {
       // Arrange
       mockZendeskService(toNationalId, fromNationalId, {
diff --git a/libs/auth-api-lib/src/lib/delegations/admin/delegation-admin-custom.service.ts b/libs/auth-api-lib/src/lib/delegations/admin/delegation-admin-custom.service.ts
index 212577223606..5cbb11d913b5 100644
--- a/libs/auth-api-lib/src/lib/delegations/admin/delegation-admin-custom.service.ts
+++ b/libs/auth-api-lib/src/lib/delegations/admin/delegation-admin-custom.service.ts
@@ -29,6 +29,7 @@ import { DelegationDelegationType } from '../models/delegation-delegation-type.m
 import { DelegationsIncomingCustomService } from '../delegations-incoming-custom.service'
 import { DelegationValidity } from '../types/delegationValidity'
 import { ErrorCodes } from '@island.is/shared/utils'
+import { Op } from 'sequelize'
 
 @Injectable()
 export class DelegationAdminCustomService {
@@ -303,6 +304,36 @@ export class DelegationAdminCustomService {
       this.namesService.getPersonName(delegation.toNationalId),
     ])
 
+    const existingDelegation = await this.delegationModel.findOne({
+      where: {
+        [Op.or]: [
+          {
+            fromNationalId: delegation.fromNationalId,
+            toNationalId: delegation.toNationalId,
+          },
+          {
+            referenceId: delegation.referenceId,
+          },
+        ],
+      },
+    })
+
+    if (existingDelegation) {
+      // Throw error if reference ID already exists in db.
+      if (existingDelegation.referenceId === delegation.referenceId) {
+        throw new BadRequestException({
+          message: 'Delegation with the same reference id already exists',
+          error: ErrorCodes.REFERENCE_ID_ALREADY_EXISTS,
+        })
+      } else {
+        // Throw generic error if delegation already exists.
+        throw new BadRequestException({
+          message: 'Could not create delegation',
+          error: ErrorCodes.COULD_NOT_CREATE_DELEGATION,
+        })
+      }
+    }
+
     return this.sequelize.transaction(async (transaction) => {
       return this.delegationModel.create(
         {
diff --git a/libs/auth-api-lib/src/lib/delegations/models/delegation.model.ts b/libs/auth-api-lib/src/lib/delegations/models/delegation.model.ts
index a750ab22c691..ac49efc3cae1 100644
--- a/libs/auth-api-lib/src/lib/delegations/models/delegation.model.ts
+++ b/libs/auth-api-lib/src/lib/delegations/models/delegation.model.ts
@@ -84,7 +84,7 @@ export class Delegation extends Model<
     allowNull: true,
   })
   @ForeignKey(() => Domain)
-  domainName?: string
+  domainName?: string | null
 
   /**
    * ReferenceId is a field for storing a reference to the zendesk ticket id
diff --git a/libs/portals/admin/delegation-admin/src/constants/errors.ts b/libs/portals/admin/delegation-admin/src/constants/errors.ts
index ca4a8a0d020b..ac8b1476dc6e 100644
--- a/libs/portals/admin/delegation-admin/src/constants/errors.ts
+++ b/libs/portals/admin/delegation-admin/src/constants/errors.ts
@@ -12,4 +12,6 @@ export const FORM_ERRORS: Record<ErrorCodes, MessageDescriptor> = {
   [ErrorCodes.INPUT_VALIDATION_SAME_NATIONAL_ID]: m.sameNationalIdError,
   [ErrorCodes.INPUT_VALIDATION_INVALID_PERSON]: m.validPersonError,
   [ErrorCodes.INVALID_DATE_FORMAT]: m.invalidDateFormatError,
+  [ErrorCodes.COULD_NOT_CREATE_DELEGATION]: m.couldNotCreateDelegationError,
+  [ErrorCodes.REFERENCE_ID_ALREADY_EXISTS]: m.referenceIdAlreadyExistsError,
 }
diff --git a/libs/portals/admin/delegation-admin/src/lib/messages.ts b/libs/portals/admin/delegation-admin/src/lib/messages.ts
index 9bd273bf7545..990743fc4a0d 100644
--- a/libs/portals/admin/delegation-admin/src/lib/messages.ts
+++ b/libs/portals/admin/delegation-admin/src/lib/messages.ts
@@ -159,4 +159,12 @@ export const m = defineMessages({
     id: 'admin.delegationAdmin:invalidDateFormatError',
     defaultMessage: 'Dagsetning er ekki á réttu sniði',
   },
+  couldNotCreateDelegationError: {
+    id: 'admin.delegationAdmin:couldNotCreateDelegationError',
+    defaultMessage: 'Ekki tókst að skrá umboð',
+  },
+  referenceIdAlreadyExistsError: {
+    id: 'admin.delegationAdmin:referenceIdAlreadyExistsError',
+    defaultMessage: 'Númer máls í Zendesk er þegar til',
+  },
 })
diff --git a/libs/services/auth/testing/src/fixtures/types.ts b/libs/services/auth/testing/src/fixtures/types.ts
index 8b96406dd1e3..60f34ce8e514 100644
--- a/libs/services/auth/testing/src/fixtures/types.ts
+++ b/libs/services/auth/testing/src/fixtures/types.ts
@@ -37,11 +37,14 @@ export type CreateCustomDelegationScope = Optional<
 export type CreateCustomDelegation = Optional<
   Pick<
     DelegationDTO,
-    'toNationalId' | 'fromNationalId' | 'fromName' | 'referenceId'
+    | 'toNationalId'
+    | 'fromNationalId'
+    | 'fromName'
+    | 'referenceId'
+    | 'domainName'
   >,
-  'toNationalId' | 'fromNationalId' | 'fromName' | 'referenceId'
+  'toNationalId' | 'fromNationalId' | 'fromName' | 'referenceId' | 'domainName'
 > & {
-  domainName: string
   scopes?: CreateCustomDelegationScope[]
 }
 
diff --git a/libs/shared/utils/src/lib/errorCodes.ts b/libs/shared/utils/src/lib/errorCodes.ts
index 54ed91b8d870..3eeb74879619 100644
--- a/libs/shared/utils/src/lib/errorCodes.ts
+++ b/libs/shared/utils/src/lib/errorCodes.ts
@@ -6,4 +6,6 @@ export enum ErrorCodes {
   INPUT_VALIDATION_SAME_NATIONAL_ID = 'INPUT_VALIDATION_SAME_NATIONAL_ID',
   INPUT_VALIDATION_INVALID_PERSON = 'INPUT_VALIDATION_INVALID_PERSON',
   INVALID_DATE_FORMAT = 'INVALID_DATE_FORMAT',
+  COULD_NOT_CREATE_DELEGATION = 'COULD_NOT_CREATE_DELEGATION',
+  REFERENCE_ID_ALREADY_EXISTS = 'REFERENCE_ID_ALREADY_EXISTS',
 }