diff --git a/cypress/integration/validation.js b/cypress/integration/validation.js
index 3eddfef..e796fcb 100644
--- a/cypress/integration/validation.js
+++ b/cypress/integration/validation.js
@@ -39,7 +39,6 @@ describe('Document validation', function() {
         );
         cy.get('#validateCodemeta').click();
 
-
         cy.get('#name').should('have.value', '');
         cy.get('#errorMessage').should('have.text', '');
     });
@@ -105,6 +104,20 @@ describe('Document validation', function() {
 
         cy.get('#errorMessage').should('have.text', 'Unknown field "foobar".');
     });
+
+    it('errors when both "type" and "@type" are present', function() {
+        cy.get('#codemetaText').then((elem) =>
+            elem.text(JSON.stringify({
+                "@context": "https://doi.org/10.5063/schema/codemeta-2.0",
+                "type": "SoftwareSourceCode",
+                "@type": "SoftwareSourceCode",
+                "name": "Conflicting types example",
+            }))
+        );
+        cy.get('#validateCodemeta').click();
+
+        cy.get('#errorMessage').should('have.text', 'Document must use either "type" or "@type", not both.');
+    });
 });
 
 describe('URLs validation', function() {
diff --git a/js/validation/index.js b/js/validation/index.js
index 4d396a3..d9a284a 100644
--- a/js/validation/index.js
+++ b/js/validation/index.js
@@ -20,7 +20,13 @@ function validateDocument(doc) {
     }
     // TODO: validate id/@id
 
-    // TODO: check there is either type or @type but not both
+    // 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(`Document must use either "type" or "@type", not both.`);
+        return false;
+    }
+
     var type = getDocumentType(doc);
     if (type === undefined) {
         setError("Missing type (must be SoftwareSourceCode or SoftwareApplication).")