Forms :: Using Form To Display Previous Entries And Add New Ones?

I'm trying to design a simple wildlife tracking DB, so that when myself or my colleagues spot an animal we can enter its ear tag number via a form. That will either bring us up a list of previous dates/locations that animal was observed, or if it's not in the DB, allow us to enter the new animal and location.

I have two main data tables:
dt_Animal- which stores the ear tag info, species, and other information about that unique animal
dt_Sightings - stores info on when/where animals been seen (linked to dt_Animal in a one-to-many relationship, of course)

What I want is that when you open up the form, you get a box (text or combo) to enter the animal's ear tag number.

- If it already exists in the DB, it brings up all the info on that animal and its previous sightings - with the option to fill out any missing info and (of course) to add a new sighting - and it WILL NOT add a new record to dt_Animal - just to dt_Sightings
- If it DOESN'T exist in the DB, it creates a new record in dt_Animal and allows you to enter any/all of the other information

Where I'm running into problems is that I don't want duplicate records in dt_Animal - each animal is unique. I haven't found a way to set up a form so that you don't get pop-up errors if you're typing in an ear tag number that's already in the system.

I've tried various tactics involving simple coding (NotInList, etc.) that I pulled out of this forum and other places, but haven't had any luck.