Drop down menu need to show not repeats of a collection

Hi, we have few collection datas on our website and one of them is "listings", where our Clinic partners are listing, for now only in Turkey. We want to link a dropdown menu on our homepage to cities where the partners are. On the list there are and will of course many clinics or other partners in the the same cities or countries, but we dont want the dropdown to show all the repeats of cities on the list, but we don`t know how to create a dropdown without this repeats and with a link to all linked city pages which needs to be created? Can you please help us to solve this problem? We have this same problem also on contact forms . Thanks in advance. Best Regards, Seda

Solution here: