Probably a dumb question but not getting answers anywhere... I'm getting an error:
"How-to: This block can only be used once." from a yoast seo plugin but is this a limitation imposed by wp or yoast?
WordPress doesn't put restrictions on third-party blocks. The only way to ensure a block can be added only once per post (or Page, or CPT) is when registering it - meaning the restriction was added by Yoast.
You could look into modifying the block yourself but there is usually a good reason when an author specifies the block can only be added once. Perhaps there is built-in Schema or it is coded in such a way that it wouldn't make sense to have more than one per page.