SXA duplicate main placeholder

Sitecore SXA & the main placeholder mystery

It sounds like a fairy tale and it almost was but a few hours were lost on this mystery so maybe writing this down can save someone some time in the future. 

What happened? We installed SXA on a XM environment with an already existing site (not using SXA, just a plain old MVC site). The idea was to use SXA for future sites that are being build on the same environment. But of course, we had to test if the current site was still working with this new framework installed.

The first tests seemed fine. The existing site was still working and we couldn't find any issues in the logs. But then we noticed that one placeholder was not working anymore in the Experience Editor. The site had a placeholder called "main" (as many sites probably do) which the editors will mainly utilize when creating new pages. The issue stayed under the radar when editing existing pages or even adding some new components as those are usually put in other placeholders.

So we faced an issue with the "main" placeholder. The person debugging this had knowledge of the current implementation but no real experience with SXA and was a bit lost. Although if you looked carefully at the placeholder definition item, Sitecore did give a pretty decent clue: 

Duplicate placeholder keys

Sitecore is telling us here that the key value should be unique. Which make sense of course. So we have to find out why this is suddenly happening and of course we should look into SXA. And yes, SXA als has by default a main placeholder for the MVC version: /sitecore/layout/Placeholder Settings/Foundation/Experience Accelerator/MVC/main.  As we have will only be using the headless version the easy fix was to change the placeholder key in this SXA item. 

Note that other placeholders keys are added as well, like header, footer, head...  so you might encounter this issue on those as well.

It appeared to be an easy fix for a trivial issue, but for those who are not acquainted with SXA this hopefully helps if you ever face this problem.