A save refused with "already exists" has hit one of a short list of uniqueness rules. Each has a scope, and the scope tells you where to look for the other copy. Some fields that look like keys are not checked at all, and they are here too: "why was this allowed" is as common a question as "why was it refused".
Property ID: not unique
The Property ID is your own reference for the account an address belongs to, not a name for the address. It takes letters as well as digits, and it may repeat: give the same id to every address on one account, to a whole estate, or to sixteen streets under one property administrator. Nothing is refused.
The id shows in front of the name throughout the app (12345 - Østbanegade 55), and Search on Locations matches it, so an id shared by several addresses finds all of them at once. Sorting on it keeps plain numbers in numeric order and puts ids containing letters after them. An empty field stays empty and clashes with nothing.
Contacts on an address: one row each
The same e-mail address, or the same phone number, may not appear twice in one address's contact list. A second row with either is refused with Location: the email address or phone number is already in use. Edit or remove the duplicate row.
That is the whole rule. The same person may be a contact on any number of addresses, and the contact for any number of customers: one property administrator can be the billing contact for fifty of your customers under a single e-mail address and a single phone number.
Notification e-mails are checked as you add them, per address: This email is already added to the notification list.
Customer number: your business
Each of your customers has its own Customer number. A clash is refused with Customer with the same Customer Number already exists (import: Customer number already exists.). Search Customers for the number and use that customer, or accept the next free number the form suggests. Two businesses can use the same numbers.
Tax ID and EAN: not unique
Several customers may share a CVR or an EAN, in your business and across businesses; a property administrator billing many customers under one number is the everyday case. Saving is never refused for either. Only the Find button on the customer form checks the CVR: it skips the register lookup with Customer already exists. when you already have a customer on it. Type the details by hand and the save goes through. Your own business's tax ID is one per business.
Users: the e-mail address, not the number
An e-mail address belongs to one user account across all of SnowManager. You sign in with it, so it has to stand alone: User already exists or This email is already registered. If you cannot see the user, they belong to another business; use a different address, or have the existing account given access.
A mobile number does not have to be unique. Several accounts may carry the same one, which is the ordinary case for a crew sharing a handset or an office number left on a few accounts. The number decides where a text-message code is delivered and nothing else, so sharing it opens no account to anybody. Changing your own number still needs the six-digit code sent to it, see account security.
Names in your business
Tag, drawing tool (per tool type) and role names must be unique in your business. A work type whose name exists is reused, not refused. Route, zone, group and rule names may repeat.