Duplicate Accounts when doing an report

Duplicate Accounts when doing an report

Hi,

I have done a report that shows all accounts which haven't got any activities tasks or calls(so we can track them and call them). My problem is that I have accounts appearing 3 or 4 times in the table and I can't find why. 
Is it because it also looks for Contacts without any tasks ? 

Many thanks for your answer.
Regards,

Florian