Aggregate an indicator by SUM duration on unique records

[Blair5]

I've created an indicator on the TASK_CI table and want to sum the duration of the unique TASK's open duration. Currently, I can only SUM the total records of the TASK_CI.

For example, 2 tasks (incidents) are created both totaling 60 minutes each for a total of 120 min.

However, if each task has 5 CIs associated with them, the duration is summed by total rows (10) in the TASK_CI table and not by the 2 unique TASK (incident) records. 

See below for the script I'm using for the SUM of the duration - pretty basic. I understand it calculates the duration for each record, but I want to exclude the record if it already appears in the calculation. 

Any help is appreciated:

var diff = function(x, y) {
    return y.dateNumericValue() - x.dateNumericValue();
};
var min = function(x, y) {
    return diff(x, y) / (1000 * 60);
};

min(current.task.opened_at, current.task.closed_at);