Extensions | An item with the same key has already been added

If you work with Extensions in Dynamics NAV there is a fair chance that you’ve seen this message.

Today I got it again, waisted half an hour of my live and decided to blog about it.

What happened is that I have a Page Extension with the same variable name as already exists in my C/Side page.

I’m working with NAV2018 CU17 and the compiler did not complain about this and the publishing process went fine. When you try to open the page the client crashes and this error is what you get in the Windows Event log.

Hope this helps someone.