Sub-theme adding prefix to all block machine names On my site, I have created a sub-theme of a sub-theme using this as a guide to set this up https://www.drupal.org/docs/8/theming-drupal-8/creating-a-drupal-8-sub-theme-or-sub-theme-of-sub-theme and it does work with carrying over all of the css/js/regions but for some reason, it is renaming all of the blocks with the prefix of the sub-theme. 

So, for example, I have a block with a machine name of mainnavigation when I install/set as default the sub-theme it will change this block to theme3_mainnavigation. 

I have tried removing the block and placing it back in with the same name of mainnavigation but I run into an issue where a message will appear that says this machine name already exists. 

Does anyone know how to get around this so the sub-theme will use the machine names of already existing names instead of creating a prefix?

Thanks for any help!
You can't. 

This behavior is core functionality from the Block module that's enforcing its data model. Each time you place a block, you're creating a unique Block entity that has a theme property tied to it. Thus, the mainnavigation block entity can't simultaneously be used on the default theme as well as theme3. 

This is why Drupal defaults to generating ${theme_machine_name}_${block_machine_name} so the user doesn't have to think about it.