DB error: already exists on contribute page
It means you a trying to create a new account with an email that is already in use for another account.

Some CMS (drupal at least!?) don't allow two separate accounts with the same email, hence the (rather not too explicit) error message. 

The simple solution is either to change the existing account so it uses another email address or create the new account with an email that isn't used already.

The not so simple but would be awesome for your karma solution is to contribute a patch to display an "end user friendly" error message and the option to put another email in the form if its invalid

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