Duplicate listing error

Hello,
I just tested registration on my link cataloque and I have points:

1) What check wordpress during the registration for duplication? – username, email, custom field (url)?

2) If I have tested registration and something was duplicated (username, email, custom field – url). There was error message and I expect that the listing will not be send, but it is. So if there is error message that something is doubled, the listing is still published??

Thanks,
Radek