Invalid HTML when a widget set contains more than one custom markup

Hi there

I have something to report. On the website of my brother, I saw that the outputted HTML contains errors, mainly because there are multiple IDs of value "widgets-element-widgets_custom-markup".

This happens, when I add more than one "custom markup" item to a set. The set itself is custom as well. Every custom markup has the same ID and that proves invalid against the W3C Validator.

The problem is the line 1158 inside widgets.module.

I have attached a screenshot of what I mean.