Count unique values present across months

kinkate18nic 
Yes, it will be counted as 1. But you can filter it out using either of the folowing depending on whether the blank is acually BLANK () or just empty string ""

Unique AlertID Count =
VAR CurrentDate =
    MIN ( 'Table'[Event Date] )
VAR CurrentIDs =
    FILTER ( VALUES ( 'Table'[AlertID] ), 'Table'[AlertID] <> BLANK () )
VAR PreviousIDs =
    DISTINCT (
        SELECTCOLUMNS (
            FILTER ( ALLSELECTED ( 'Table' ), 'Table'[Event Date] < CurrentDate ),
            "AlertID", 'Table'[AlertID]
        )
    )
RETURN
    COUNTROWS ( EXCEPT ( CurrentIDs, PreviousIDs ) )


Unique AlertID Count =
VAR CurrentDate =
    MIN ( 'Table'[Event Date] )
VAR CurrentIDs =
    FILTER ( VALUES ( 'Table'[AlertID] ), 'Table'[AlertID] <> "" )
VAR PreviousIDs =
    DISTINCT (
        SELECTCOLUMNS (
            FILTER ( ALLSELECTED ( 'Table' ), 'Table'[Event Date] < CurrentDate ),
            "AlertID", 'Table'[AlertID]
        )
    )
RETURN
    COUNTROWS ( EXCEPT ( CurrentIDs, PreviousIDs ) )