I don’t have code for you but what you’d have to do is this.
Create an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/
In this filter you would get the value of the field saved in this post then you would do a query of other posts that have the same value and if you find one they you’d update the acf field on that other post. You would need to continue doing this until you don’t fond and posts with the same value.
