How to send task to each department user once using Multi-instance

Hi Team,
I need to send a task to multiple departments,So I am using multi-instance process.I am defining department names Using TaskListiner Class as below:

public class Departments implements TaskListener{

public void notify(DelegateTask delegateTask) {
	List<String> departments = new ArrayList<>();

If I give the loop cardinality as 3 Then each Dept user is getting 3 times.How can I assign to 3 department users only once.
Please suggest me.
Thanks In Advance


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