MultiIndexBlock::init has a for-loop for initializing indexes in-memory and on disk. In the for-loop we call IndexCatalog::prepareSpecForCreate to check for errors: bad spec, already exists.
The first loop is fine, and sets up the index. The second loop will get an IndexAlreadyExists error from IndexCatalog::prepareSpecForCreate. This error will not get swallowed – we do swallow IndexAlreadyExists errors encountered earlier in the command. The user will get an IndexAlreadyExists error and no index will have been created.