remove circular dependency between measures

These look like calculated columns, not measures, but you should be able to remove the circular dependency error by changing the initial column to

HasLossAndGain =
VAR ThisIncidentID = My_Main_Table[Incident ID]
VAR HasLoss =
    CALCULATE (
        COUNTROWS ( 'My_Main_Table' ),
        'My_Main_Table'[Incident ID] = ThisIncidentID,
        'My_Main_Table'[Financial Impact Type] = "Loss",
        REMOVEFILTERS ( 'My_Main_Table' )
    ) > 0
VAR HasGain =
    CALCULATE (
        COUNTROWS ( 'My_Main_Table' ),
        'My_Main_Table'[Incident ID] = ThisIncidentID,
        'My_Main_Table'[Financial Impact Type] = "Gain",
        REMOVEFILTERS ( 'My_Main_Table' )
    ) > 0
RETURN
    HasLoss && HasGain