Dealing with granularity - best practice

clay_75 - If you would just like to find the Cluster Duration SUMMARIZE will work, you just need to create it as a virtual table, inside a variable (VAR) in a measure, then iterate through that varible to return it for each Task Reference.

I've amended your DAX slightly below, but you'll see from the screenshot this is tested and works:

Cluster_Duration = 
VAR _table =
    SUMMARIZE (
        VALUES ( 'Table (7)'[Task Reference] ),
        "Duration Mins", MIN ( 'Table (7)'[Cluster Duration] )
    )
RETURN
    SUMX ( _table, [Duration Mins] )

If this works for you, please accept it as the solution.