In some versions of dotnetopenauth you can also get:

This message has already been processed. This could indicate a replay attack in progress.

if your maxAuthenticationTime value is too low (which is obviously not related to the error in any way - but that is a different issue). I experienced this just today.

To increase this value, edit the config entry as shown at http://dotnetopenauth.net:8000/wiki/CodeSnippets/Configuration (I suggest setting to 0:10).

In some versions of dotnetopenauth you can also get:

This message has already been processed. This could indicate a replay attack in progress.

if your maxAuthenticationTime value is too low (which is obviously not related to the error in any way - but that is a different issue). I experienced this just today.

To increase this value, edit the config entry as shown at http://www.dotnetopenauth.net/developers/help/configuration-options/ (I suggest setting to 0:10).