Cannot download email or the same email is repeatedly downloaded

This is a real gem of a topic, because it plagues many Thunderbird users, but unfortunately rarely does it actually involve Thunderbird, except as the hapless recipient of the blame.

The first port of call in these circumstances is your anti virus program.  I must sound some what bigoted with my continual rant to people to 'check their anti virus program if things are not working correctly'.  But I have linked to some posts in three anti virus support forums that might just light the way.

Nortons

Comcast customers prick your ears up now.  Notron 360 and other Norton anti virus programs will demonstrate this problem;
"same email is repeatedly downloaded"  The Symantec site advises users to simply turn off mail scanning   They do latter suggest turning it back on, but just how necessary was it in the first place if you get to do all these virus removal actions with it turned off.

{Edit 4 August 2012:  Symantec appear to have removed the article, perhaps they thought it embarrassing that their premier product just kept downloading mail over and over.  If anyone is interested the article number was/2003013115102706}

While I was there I had a bit of a look around an found this little gem in September 2011 "Are you using Port 25 to send your outgoing messages?  Norton does not support scanning on ports other than this, so if your ISP requires a different port that may cause an issue like this."  Now port 25 has not been the recommended port for a lot of years now, many ISP block port 25 so very few of the free mail providers use it.

One of the really interesting things here is that Comcast actively promote Norton 360 to their customers, even bundling it with internet packages but their outgoing port is 587 so the product they are bundling does not scan outgoing mail to their own servers.

Then there is the issue of IMAP mail accounts. In April of 2011 this post indicates the anti SPAM feature does not work with IMAP accounts at all

McAfee

The McAfee support forum is remarkably devoid of these problems.  But one issue of particular interest is the fact that McAfee only moves SPAM massages to the SPAM folder for the first account processed in a get all mail

The 2009 version of Virus scan does not scan attachments until you save or open them.  Did it change for latter versions? I will have to wait until someone tells me I suppose

Avast (See I read my comments sometimes)

Issues with Avast are numerous, and come from the fact that the anti virus program simply can't cope with secured connections. Thunderbirds' account setup wizard will set up the most secure connection it can. Thus we have something of a conflict. Thunderbird is secure, avast can't work. This is  a huge weakness in Avast. Other anti virus scanners do not suffer with this significant flaw.  However Avast do offer instructions on how to set up their anti virus program to work with Thunderbird.  They are for an older version, but hey I don't get to update their support articles.

Sudden failure to get mail.: I will not comment, but refer you to the source  but it would appear that when things go bad they go very bad and an uninstall and reinstall is required.

Avast also has an auto block option on it's firewall.  I have never used this product, but it sure would explain why people have problems with it.  It makes it's rules based on trusted sources. Avast don't mention what trusted is, but I have a feeling it is Microsoft.  The net result is you will probably find you need to fine turn this expert systems assumptions.  Luckily they provide instructions, as most questions on their support forum about firewall blocking simply go unanswered.

I think avast suffers from the same problem as Nortons, and once it detects a virus on the server, it just dies, and nothing downloads.  Unlike Nortons, I can't find a reference on their support forum, so here goes with some basic fix it yourself instructions. Turn off scanning of mail in Avast and then log into your web mail and delete the offending message.

You may or may not decide to turn the email scanner back on.  Personally I doubt the need for an email scanner and have posted about that already.