table row repetitions in task

Hi,

I'm trying to create a task that goes over all table rows in random order and without repetitions 
The table has 15 rows so I've implemented the block as following:



block block_name
 message some_message
 tasklist
   task_name 15 all_before_repeat no_repeat
 end
When I run the experiment, I notice that sometimes I get the same task row more than once 

How can I solve this problem?

Thank you