1) It depends on where the fields are. If the fields are on another options page then they will overwrite existing values. If your goal is to have 2 places to edit the same value then this will work, however, the field keys will be different and this can cause problems if the data in the fields is different. If your goal is to use the exact same field in multiple places then you’d be better off defining the field group using PHP.
On the other hand if the values in some other place, for example on a post, then they can have the same name.
Here are the rules, as it were. Any field name needs to be unique if they are located in the same place it will hold unique information. These are the places where you’ll run into trouble with 2 fields of the same name
A) Any field on any options page, this would be the same place
B) Two fields on the same Taxonomy/Term
C) Two fields on the same Post
2) You need to include the acf_form_head() call before the the wp_head() call. More of a WP thing then anything else, but you page must have a header. You’ll probably want to create a custom header and footer for this page if it does not look like the rest of your site. See the WP get_header() and get_footer() function for information on creating multiple header/footer files.
3) If the field group is created on one site it will only be usable on that site. ACF 5 offers a Local JSON feature that will load the field groups into any site using the same theme. https://www.advancedcustomfields.com/resources/local-json/. You can also define multiple load points, so for instance you could have an “Admin” theme where you create field groups and then set a load point to load them into other themes on the installation.
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