Separate files from the very first document
The mix-ups in multi-dog homes start with one shared folder that was meant to be temporary. Give each dog a fully separate file from day one, whether that is a folder, a binder section with its own divider that nothing crosses, or a digital folder named for that dog alone. Resist the household envelope for vet receipts: a receipt that names the practice and a total but not the dog will be filed wrong eventually, so write the dog's name on it yourself at the desk before you leave.
Label each file with three identifiers
Put the dog's name, a clear photo and the microchip number on the front of every file. Names get shortened, dogs of the same breed look alike on paper descriptions, and paperwork from a practice may carry a registered name the family never uses at home. The photo settles identity at a glance for a sitter or family member, and the microchip number is the one identifier that cannot be confused with a housemate's, which makes it the anchor when two files are open side by side.
File on arrival, checking the name first
Most cross-filing happens in a weekly paperwork batch done from memory. Instead, file each document on the day it arrives, reading the name printed on it before it goes anywhere, and query any document that names no dog at all while the appointment is still fresh. Vaccination cards, test results, weight notes and insurance letters each belong to one animal only, and a thirty-second check at filing saves the far worse discovery that a boarder or vet has been reading the wrong dog's history. Colour-coding the spines or folder tabs helps younger family members file correctly at a glance, but treat colour as a backup to the printed name, never as the only label. Keep paperwork arriving for a new puppy in its own starter file from the first vet visit, rather than parking it in an adult dog's folder until it has enough pages of its own.
Keep a one-page household index, not shared records
A single index sheet for the household is genuinely useful, provided it points rather than contains: each dog's name, file location, vet practice, insurer and microchip number, with renewal dates listed per dog. It lets anyone see at a glance whose booster falls next without opening a file, while the underlying records stay strictly individual. Update the index whenever a dog joins or leaves the household, since a stale index that still lists a rehomed dog invites exactly the confusion the system exists to prevent.
What to do when you find a past mix-up
If you discover a document filed under the wrong dog, move it, then add a short dated note in both files recording the correction, so a future reader does not reintroduce the error from an old copy. Where the mix-up reached a vet, groomer or insurer, tell them directly and ask them to correct their copy too. Keeping an individual Pet Passport on KinsLe for each of your dogs builds that separation into the structure itself, with every profile carrying only its own dog's records, photo and chip details.