Force WP to ignore multiple whitespaces

Not aware of a global setting to do this but you have a few options.

Use add_filter( ‘the_content’, ‘filter_function_name’ ) and replace the double spaces. Generally a bad idea because you’d have to account for the valid use of double spaces.

Install a plugin like ‘After the Deadline’ or similar that shows spelling and typo errors. Of course you can’t force the user to accept the changes/corrections.