By default, when people visit your website by clicking on a special link in an email message sent by Gnosis, they are in effect automatically logged in to their personal portal on your website as long as you have left this feature enabled in your system. This means, if they choose to donate in that web session, the website will know who they are, will pre-fill most of their donation information, and will automatically add their donation to their existing Gnosis record.
When someone visits the website organically (i.e.: using the web address printed in a physical mailer or typing in the URL to a browser) however, they are anonymous to the website, so, when they go to donate, Gnosis will attempt to match them to an existing record using the supplied first name and email address. If there is no match in the database, Gnosis will create a brand new record for them and attach their donation to that new record
Using this process, there are going to be duplicated records to be dealt with. This will be dealt with as long as someone in the organization is charged with the task of consolidating those duplicates using the Gnosis duplicate merge functions.