diff --git a/cypress/integration/validation.js b/cypress/integration/validation.js
index e796fcb..3670f89 100644
--- a/cypress/integration/validation.js
+++ b/cypress/integration/validation.js
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2020-2021  The Software Heritage developers
+ * Copyright (C) 2020-2025  The Software Heritage developers
  * See the AUTHORS file at the top-level directory of this distribution
  * License: GNU Affero General Public License version 3, or any later version
  * See top-level LICENSE file for more information
@@ -105,7 +105,7 @@ describe('Document validation', function() {
         cy.get('#errorMessage').should('have.text', 'Unknown field "foobar".');
     });
 
-    it('errors when both "type" and "@type" are present', function() {
+    it('errors on having both "type" and "@type"', function() {
         cy.get('#codemetaText').then((elem) =>
             elem.text(JSON.stringify({
                 "@context": "https://doi.org/10.5063/schema/codemeta-2.0",
@@ -317,6 +317,42 @@ describe('Things or URLs validation', function() {
         cy.get('#errorMessage').should('have.text', '"license" must be an URL or a CreativeWork/SoftwareSourceCode/SoftwareApplication object, not: "Copyright 2021 Myself"');
     });
 
+    it('errors on having both "id" and "@id"', function () {
+        cy.get('#codemetaText').then((elem) =>
+            elem.text(JSON.stringify({
+                "@context": "https://doi.org/10.5063/schema/codemeta-2.0",
+                "@type": "SoftwareSourceCode",
+                "author": {
+                    "id": "http://example.org/~jdoe",
+                    "@id": "http://example.org/~jdoe",
+                    "@type": "Person",
+                    "name": "Jane Doe",
+                },
+            }))
+        );
+        cy.get('#validateCodemeta').click();
+
+        cy.get('#errorMessage').should('have.text', '"author" must use either "id" or "@id", not both.');
+    });
+
+    it('errors on having both "type" and "@type"', function () {
+        cy.get('#codemetaText').then((elem) =>
+            elem.text(JSON.stringify({
+                "@context": "https://doi.org/10.5063/schema/codemeta-2.0",
+                "@type": "SoftwareSourceCode",
+                "author": {
+                    "id": "http://example.org/~jdoe",
+                    "type": "Person",
+                    "@type": "Person",
+                    "name": "Jane Doe",
+                },
+            }))
+        );
+        cy.get('#validateCodemeta').click();
+
+        cy.get('#errorMessage').should('have.text', '"author" must use either "type" or "@type", not both.');
+    });
+
     it('errors on wrong type', function() {
         cy.get('#codemetaText').then((elem) =>
             elem.text(JSON.stringify({
diff --git a/js/validation/index.js b/js/validation/index.js
index d9a284a..815a3b7 100644
--- a/js/validation/index.js
+++ b/js/validation/index.js
@@ -21,7 +21,7 @@ function validateDocument(doc) {
     // TODO: validate id/@id
 
     // Ensure either "type" or "@type" is used, but not both
-    const typeKeys = ['type', '@type'];
+    const typeKeys = ["type", "@type"];
     if (typeKeys.filter(k => Object.prototype.hasOwnProperty.call(doc, k)).length > 1) {
         setError(`Document must use either "type" or "@type", not both.`);
         return false;
diff --git a/js/validation/things.js b/js/validation/things.js
index 53cc567..f097bbf 100644
--- a/js/validation/things.js
+++ b/js/validation/things.js
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2020-2021  The Software Heritage developers
+ * Copyright (C) 2020-2025  The Software Heritage developers
  * See the AUTHORS file at the top-level directory of this distribution
  * License: GNU Affero General Public License version 3, or any later version
  * See top-level LICENSE file for more information
@@ -67,8 +67,19 @@ function validateThingOrId(parentFieldName, typeFieldValidators, doc) {
 //
 // typeFieldValidators is a map: {type => {fieldName => fieldValidator}}
 function validateThing(parentFieldName, typeFieldValidators, doc) {
-    // TODO: check there is either id or @id but not both
-    // TODO: check there is either type or @type but not both
+    // Ensure either "id" or "@id" is used, but not both
+    const idKeys = ["id", "@id"];
+    if (idKeys.filter(k => Object.prototype.hasOwnProperty.call(doc, k)).length > 1) {
+        setError(`"${parentFieldName}" must use either "id" or "@id", not both.`);
+        return false;
+    }
+
+    // Ensure either "type" or "@type" is used, but not both
+    const typeKeys = ["type", "@type"];
+    if (typeKeys.filter(k => Object.prototype.hasOwnProperty.call(doc, k)).length > 1) {
+        setError(`"${parentFieldName}" must use either "type" or "@type", not both.`);
+        return false;
+    }
 
     var acceptedTypesString = Object.keys(typeFieldValidators).join('/');
 
@@ -196,6 +207,7 @@ function validateReview(fieldName, doc) {
     return validateThingOrId(fieldName, {"Review": reviewFieldValidators}, doc);
 }
 
+// Define validators for each field of each type of Thing
 
 var softwareFieldValidators = {
     "@id": validateUrl,
@@ -323,7 +335,6 @@ var personFieldValidators = {
     "url": validateUrls,
 };
 
-
 var organizationFieldValidators = {
     "@id": validateUrl,
     "id": validateUrl,