Alex, it seems the "removeDuplicate" option works for each column independently. However, there's a workaround to make it work across columns by concatenating several columns together. And then use a column's "formatValue" option to show its self value only:
    //MyReport.php
    ...
    ->pipe(new \koolreport\processes\Map(array(
        "{value}" => function($row) {
            $row["starttime"] = $row["startdate"] . "||" . $row["starttime"]; //concatenate the date and time columns together
            return $row;
        }
    )))
    //MyReport.view.php
    $columns = $this->buildColumns();
    $columns["starttime"]["formatValue"] = function($value, $row, $cKey) {
        $timePart = explode("||", $value)[1];
        return $timePart;
    }
    Table::create([
        "dataStore"=>$this->dataStore('Direct'),
        "columns"=>$columns,
        "removeDuplicate" => ["startdate", "starttime"],
        "cssClass"=>["table" =>"table table bordered table-striped", "th" => ""],
        ],
    );            
Pls try it and let us know if there's any problem. Cheers,