database update how to prevent duplicates

Hi

database schema has items

login
groupid

Now when database is updated how can I check if there is entry already
with same information example:

new item is created with values login=doe groupid=10 and if such item
exists already on database then error is returned ?

I believe on model you can do something like this, correct ?

I know on model you can check if login=doe is already there but I need
to make sure that if both conditions
apply at the same time.

Note the commentary about creating a unique index if you want this to
be safe in the face of concurrency.