pppoe public ip pool assignment

Hello,

I am running pppoe server on mikrotik 6.21
Recently I acquired public ip pool from my local ISP to provide it to users. All thing working ok so far.
the problem is every time the user connect to PPPoE server , he gets the same IP address as last connection IP address, while the IP pool has unused IP. Mikrotik dont have any dhcp server configured, so its pool base dynamic assignment. How can i force that user must get different IP each time.