diff --git a/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/StopBuyerIfSameAsSeller/index.tsx b/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/StopBuyerIfSameAsSeller/index.tsx
new file mode 100644
index 000000000000..02058dbd6d1e
--- /dev/null
+++ b/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/StopBuyerIfSameAsSeller/index.tsx
@@ -0,0 +1,18 @@
+import { FieldBaseProps } from '@island.is/application/types'
+import { FC } from 'react'
+import { doSellerAndBuyerHaveSameNationalId } from '../../utils'
+
+export const StopBuyerIfSameAsSeller: FC<
+  React.PropsWithChildren<FieldBaseProps>
+> = (props) => {
+  const { application, setBeforeSubmitCallback } = props
+
+  setBeforeSubmitCallback?.(async () => {
+    if (doSellerAndBuyerHaveSameNationalId(application.answers)) {
+      return [false, '']
+    }
+    return [true, null]
+  })
+
+  return <></>
+}
diff --git a/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/index.ts b/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/index.ts
index 9afdf0c6eec2..60e8c6a65c3c 100644
--- a/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/index.ts
+++ b/libs/application/templates/aosh/transfer-of-machine-ownership/src/fields/index.ts
@@ -1,3 +1,4 @@
 export { ApplicationStatus } from './ApplicationStatus'
 export { MachinesField } from './MachinesField'
 export { Review } from './Review'
+export { StopBuyerIfSameAsSeller } from './StopBuyerIfSameAsSeller'
diff --git a/libs/application/templates/aosh/transfer-of-machine-ownership/src/forms/TransferOfMachineOwnershipForm/InformationSection/buyerSubSection.ts b/libs/application/templates/aosh/transfer-of-machine-ownership/src/forms/TransferOfMachineOwnershipForm/InformationSection/buyerSubSection.ts
index c7f75e4884dd..80fc500d3237 100644
--- a/libs/application/templates/aosh/transfer-of-machine-ownership/src/forms/TransferOfMachineOwnershipForm/InformationSection/buyerSubSection.ts
+++ b/libs/application/templates/aosh/transfer-of-machine-ownership/src/forms/TransferOfMachineOwnershipForm/InformationSection/buyerSubSection.ts
@@ -1,12 +1,15 @@
 import {
+  buildAlertMessageField,
+  buildCustomField,
   buildMultiField,
   buildNationalIdWithNameField,
   buildPhoneField,
   buildSubSection,
-  buildSubmitField,
   buildTextField,
 } from '@island.is/application/core'
 import { information } from '../../../lib/messages'
+import { FormValue } from '@island.is/application/types'
+import { doSellerAndBuyerHaveSameNationalId } from '../../../utils'
 
 export const buyerSubSection = buildSubSection({
   id: 'buyer',
@@ -35,6 +38,19 @@ export const buyerSubSection = buildSubSection({
           width: 'half',
           required: true,
         }),
+        buildAlertMessageField({
+          id: 'buyer.alertMessage',
+          alertType: 'warning',
+          title: information.labels.buyer.alertTitle,
+          message: information.labels.buyer.alertMessage,
+          condition: (answer: FormValue) =>
+            doSellerAndBuyerHaveSameNationalId(answer),
+        }),
+        buildCustomField({
+          id: 'buyer.custom',
+          title: '',
+          component: 'StopBuyerIfSameAsSeller',
+        }),
       ],
     }),
   ],
diff --git a/libs/application/templates/aosh/transfer-of-machine-ownership/src/lib/messages/information.ts b/libs/application/templates/aosh/transfer-of-machine-ownership/src/lib/messages/information.ts
index 5678775a9c72..844d836dfa18 100644
--- a/libs/application/templates/aosh/transfer-of-machine-ownership/src/lib/messages/information.ts
+++ b/libs/application/templates/aosh/transfer-of-machine-ownership/src/lib/messages/information.ts
@@ -266,6 +266,17 @@ export const information = {
         defaultMessage: 'Staðfesta',
         description: 'Submit button for buyer',
       },
+      alertTitle: {
+        id: 'aosh.tmo.application:information.labels.buyer.alertTitle',
+        defaultMessage: 'Kennitala sú sama og hjá seljanda',
+        description: `Buyer alert title`,
+      },
+      alertMessage: {
+        id: 'aosh.tmo.application:information.labels.buyer.alertMessage',
+        defaultMessage:
+          'Seljandi og kaupandi getur ekki verið sá sami. Vinsamlega skráðu nýja kennitölu.',
+        description: `Buyer alert message`,
+      },
     }),
     buyerOperators: defineMessages({
       title: {
diff --git a/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/doSellerAndBuyerHaveSameNationalId.ts b/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/doSellerAndBuyerHaveSameNationalId.ts
new file mode 100644
index 000000000000..3113fbe45951
--- /dev/null
+++ b/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/doSellerAndBuyerHaveSameNationalId.ts
@@ -0,0 +1,17 @@
+import { getValueViaPath } from '@island.is/application/core'
+import { FormValue } from '@island.is/application/types'
+
+export const doSellerAndBuyerHaveSameNationalId = (answers: FormValue) => {
+  const buyerNationalId = getValueViaPath(
+    answers,
+    'buyer.nationalId',
+    '',
+  ) as string
+  const sellerNationalId = getValueViaPath(
+    answers,
+    'seller.nationalId',
+    '',
+  ) as string
+
+  return buyerNationalId === sellerNationalId
+}
diff --git a/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/index.ts b/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/index.ts
index e5350d91740c..d4364c5a10a6 100644
--- a/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/index.ts
+++ b/libs/application/templates/aosh/transfer-of-machine-ownership/src/utils/index.ts
@@ -15,6 +15,7 @@ export { getReviewSteps } from './getReviewSteps'
 export { hasReviewerApproved } from './hasReviewerApproved'
 export { getApproveAnswers } from './getApproveAnswers'
 export { getRejecter } from './getRejecter'
+export { doSellerAndBuyerHaveSameNationalId } from './doSellerAndBuyerHaveSameNationalId'
 
 export const getChargeItemCodes = (
   application: Application<FormValue>,