What you need to do is create an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/
In this filter
1) Check that it’s the right post type
2) See if the checkbox is checked for this post
3) If the checkbox is checked, do a query for posts that also have the checkbox checked and exclude the current post.
4) If any posts are returned in the query then update the field uncheck it https://www.advancedcustomfields.com/resources/update_field/
Sorry I can’t provide any code, but what looks like 4 simple steps would take me quite some time to code.
