Check if available values of a list-field is already used by other node of content type
you need to build a custom module in which you implement hook_form_FORM_ID_alter. since we're dealing with node create/edit forms, try yourmodule_form_yourcontenttype_node_form_alter(). 

(you could go for hook_form_alter(), but that would yield less performance, especially since you have a query to run.)

your implementation should include a query for that specific field, probably the table field_data_field_yourfield. narrow it by bundle and if it returns any entity_id, that's the one you're filtering out of your dropdown. 

here's a few links to help you sort things out: 


function hook_form_FORM_ID_alter():
https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_form_FORM_ID_alter/7.x
function db_select(): https://api.drupal.org/api/drupal/includes!database!database.inc/function/db_select/7.x
also look up "D7 Form API reference" (I can't post more than two links atm, sorry)


please write back if it works. good luck!

cheers, 
Luco