Warning if a class name is used multiple times

src/compiler/compile/nodes/Element.ts

@@ -268,6 +268,7 @@ export default class Element extends Node {
 		}
 
 		this.validate_attributes();
+		this.validate_classes();
 		this.validate_bindings();
 		this.validate_content();
 		this.validate_event_handlers();
@@ -470,6 +471,36 @@ export default class Element extends Node {
 		}
 	}
 
+	validate_classes() {
+		const classAttribute = this.attributes.find(
+			attribute => !attribute.is_spread && attribute.name.toLowerCase() === "class"
+		);
+
+		let value: string | true = '';
+
+		if (classAttribute) {
+			value = classAttribute.get_static_value()
+		}
+
+		const classNames = String(value).split(" ");
+
+		this.classes.forEach(class_directive => {
+			const { name } = class_directive;
+
+			name.split(",").forEach(className => {
+				if (classNames.includes(className)) {
+					this.component.warn(this, {
+						code: `class-name-multiple-attrs`,
+						message: `Class: avoid using class name '${className}' in more than one class attribute`
+					});
+
+				} else {
+					classNames.push(className);
+				}
+			})
+		})
+	}
+
 	validate_bindings() {
 		const { component } = this;