BufferGeometry: Prevented duplicate index buffers.

src/core/BufferGeometry.js

@@ -66,6 +66,8 @@ THREE.BufferGeometry.prototype = {
 			console.warn( 'THREE.BufferGeometry.addAttribute: Use .setIndex() for index attribute.' );
 			this.setIndex( attribute );
 
+			return;
+
 		}
 
 		this.attributes[ name ] = attribute;