I get a messages saying "address is already used for another property" when trying to create a property

SortScape requires the address line to be unique for each property. Generally you can get around this by calling one something like “7 High St” and the other “7 High Street” for example.

One common reason to create the same address twice is when two people regularly pay for different work there (for example, the tenant pays for lawn service and the landlord pays for garden maintenance). See Handling multiple payers.