diff --git a/packages/core/css/system-classes.ts b/packages/core/css/system-classes.ts
index cff35f4953..3c1edadcb9 100644
--- a/packages/core/css/system-classes.ts
+++ b/packages/core/css/system-classes.ts
@@ -12,7 +12,10 @@ export namespace CSSUtils {
 	}
 
 	export function pushToSystemCssClasses(value: string): number {
-		cssClasses.push(value);
+		const index = cssClasses.indexOf(value);
+		if (index == -1) {
+			cssClasses.push(value);
+		}
 
 		return cssClasses.length;
 	}