Cannot define multiple ‘included’ blocks for a Concern (ActiveSupport::Concern::MultipleIncludedBlocks) with cache_classes = true

For anyone hitting the same wall to read, the solution to this is to strictly respect Rails autoloading rules. That is

1. Removing all the require / require_relative
2. Add needed paths to Rails autoload paths
3. Put files at the right places with the right names so Rails can infer where to look for code to load.

More info here: https://github.com/rails/rails/issues/15767