You might accidentally find yourself in this situation when trying to duplicate a record. You would find you are trying to change the address on a listing and it is changing the address on another listing. This is because they are attached to the same property record.
For Example: You have a listing for '1 Example Street' and when you change the address to '2 Example Street' you find another Listing you have for '1 Example Street' has also changed to '2 Example Street'.
You don't have two different listings attached to two different property records, you have two listings attached to one property Record. Any changes you make in the Property record is going across both listings.
To fix this, you need to assign one of the listings to a different Property Record. If you're unsure how to do this, check out the following article: Reassign a Listing to another Property record?