well zubi exact answer to your question is still a question in this torrent world

though there r many explanations to it.

in most of the cases it happens when someone shares the account and two people download the same torrent from same account and try to download it from 2 different locations so tracker blocks 2 different ips for same account and so this error comes.

sometime client duplicates same amount to tracker twice and so tracker rejects and gives error.

most of the time i have seen people using utorrent having this problem
on the whole utorrent is best client but see nothing is perfect.

try using azureus but not latest version as u might get confused withe latest vuse version
try Azureus_2.5.0.4a
and see if u get problem there as well.

in most of the case the basic troubleshoot method is to stop the client exit it and restart.
but in some cases u have no option but to wait.