Do not duplicate Main page content block if block is in context and block layout.

See attached patch.

To make it easier for maintainers, created an issue fork, applied the patch, and fixed some code style problems. From my testing, this does fix the problem, but leaving it as Needs review, so others also test the MR fork.

Thank you @admirlju for helping 🥰

I have reviewed and tested the MR and it does fix the problem.