Q:

What are the concerns with using an icon twice

We are designing a new website template with a global navigation and a child-site navigation. It looks like we are going to use the hamburger icon as the action button to display the menu. One idea that has come up is to use the icon twice, once in each bar, one of them displaying the global nav, one displaying the site nav. Roughly like this:

Are there any existing patterns or sites which could prove that this is a reasonable idea? Or are there specific reasons for not doing this?

A:

Using the same icon for different actions is not really good practice, especially if you don't use labels.
Maybe you can integrate your child menu in the global menu hierarchically, a bit like this:

The user now only has one menu to open and doesn't need to choose between two buttons everytime he navigates.

A:

I'd say that if possible avoiding reusing the same icon would be better. Especially when they're in close proximity. It may be that the context of the second nav being a subnav needs to be highlighted by the icon itself, or text if that's not clear enough.
This is a similar problem to something I ran into while building out the mobile functionality of the reading.ac.uk website, initially I had two of the same hamburger icon, but it tested badly and people didn't know what they did. This is what we have at the moment (2 different icons with text where size permits).
http://www.reading.ac.uk/Study/study-studysupport.aspx
Make sure you resize it down to see it.

