diff --git a/packages/happy-dom/src/custom-element/CustomElementRegistry.ts b/packages/happy-dom/src/custom-element/CustomElementRegistry.ts
index 4b37b481e..b7af253ae 100644
--- a/packages/happy-dom/src/custom-element/CustomElementRegistry.ts
+++ b/packages/happy-dom/src/custom-element/CustomElementRegistry.ts
@@ -65,6 +65,11 @@ export default class CustomElementRegistry {
 			throw new DOMException(`Custom Element: "${localName}" already defined.`);
 		}
 
+		const otherName = this.getName(elementClass);
+		if (otherName) {
+			throw new DOMException(`Custom Element already defined as "${otherName}".`);
+		}
+
 		this._registry[localName] = {
 			elementClass,
 			extends: options && options.extends ? options.extends.toLowerCase() : null
diff --git a/packages/happy-dom/test/custom-element/CustomElementRegistry.test.ts b/packages/happy-dom/test/custom-element/CustomElementRegistry.test.ts
index d6baf08fe..fc2b7020e 100644
--- a/packages/happy-dom/test/custom-element/CustomElementRegistry.test.ts
+++ b/packages/happy-dom/test/custom-element/CustomElementRegistry.test.ts
@@ -63,6 +63,11 @@ describe('CustomElementRegistry', () => {
 			expect(() => customElements.define('custom-element', CustomElement)).toThrow();
 		});
 
+		it('Throws an error if already registered under a different tag name.', () => {
+			customElements.define('custom-element', CustomElement);
+			expect(() => customElements.define('custom-element2', CustomElement)).toThrow();
+		});
+
 		it('Calls observed attributes and set _observedAttributes as a property on the element class.', () => {
 			customElements.define('custom-element', CustomElement);
 			expect(CustomElement.observedAttributesCallCount).toBe(1);