1) when i send with attachment mails avast does slow down , but it's fast to receive mails with attachment
Are you sure it's avast related?
I think your upload speed is not as great as your download one.
Can you upload a file to an ftp site and check the speed?
I think this is not avast related...
2) avast duplicates my tasks
look "VELOCISSIMO"
Can't you delete one of them?
I have this in the past but with the screen saver task... Some time later, an update corrected it.