Many Duplicate key errors in log for avatar insertions

I'm getting many avatar insertion errors like below:

PEAR Error: DB Error: already exists (INSERT INTO avatar (profile_id , original , width , height , mediatype , filename , url , created ) VALUES ( 12668 , 0 , 48 , 48 , 'image/png' , 'Twitter_16582791_dop-avatar_normal.png' , 'http://o.mrblog.nl/avatar/Twitter_16582791_dop-avatar_normal.png' , '2012-07-06 12:16:20' ) [nativecode=1062 ** Duplicate entry 'http://o.mrblog.nl/avatar/Twitter_16582791_dop-avatar_normal.png' for key 'avatar_url_key'])
2012-07-06 12:16:20 LOG_WARNING:
[o.mrblog.nl:twitterstatusfetcher.php:15448] TwitterImport Couldn't insert avatar - DB Error: already exists
2012-07-06 12:16:20 LOG_ERR: [o.mrblog.nl:twitterstatusfetcher.php:15448] twitterimport.php - Unknown error (NULL)(INSERT on avatar[profile_id='12668',original='0',width='48',height='48',mediatype='image/png',filename='Twitter_16582791_dop-avatar_normal.png',url='http://o.mrblog.nl/avatar/Twitter_16582791_dop-avatar_normal.png',created='2012-07-06 12:16:20',modified=''])

This has been the case for a long time, it does not seem to disturb the functioning of the site though.