1Password Autosave and Safari AutoFill
Some users find that 1Password occasionally offers to save a Login for a site even if they have already created one. Most often, this is a result of Safari’s AutoFill feature being enabled.
We have always recommended disabling Safari’s AutoFill feature for user names and passwords — though you can still keep it enabled for your Address Book card and other forms — because it makes things both less confusing (“I thought I already saved that!”) and more secure (Google “browser password manager security”).
That said, we are still working to see if the two can coexist more peacefully in Safari 5.1. Presently, if Safari’s AutoFill for “User names and passwords” is enabled and fills a login before 1Password, you’ll be prompted to save the Login in 1Password even if you have already done so.
In the meantime, disabling Safari’s password management should resolve the issue, keep you safer, and make your life simpler (Safari > Preferences > AutoFill > UNCHECK User names and passwords).