How to detect conflicts on the same date - i.e. people going on vacations pointing at eachother

You need to perform two joins forEach Date=>Lead ∪ Backup and Bakup∪Lead to find the intersection in each. Any join that returns match then it is true

fullOuter = 
VAR dt =
    MAX ( 'Table 1'[date] )
VAR _lead =
    MAX ( 'Table 1'[lead] )
VAR _backup =
    MAX ( 'Table 1'[backup] )
VAR tblOne_1 = { _lead }
VAR tblOne_2 = { _backup }
VAR tblTwo_1 =
    SELECTCOLUMNS (
        SUMMARIZE (
            FILTER ( ALL ( 'Table 1' ), 'Table 1'[date] = dt ),
            'Table 1'[backup]
        ),
        "@candidate", [backup] & ""
    )
VAR tblTwo_2 =
    SELECTCOLUMNS (
        SUMMARIZE (
            FILTER ( ALL ( 'Table 1' ), 'Table 1'[date] = dt ),
            'Table 1'[lead]
        ),
        "@candidate", [lead] & ""
    )
VAR tblthree_1 =
    GENERATE (
        tblTwo_1,
        VAR one = [@candidate] RETURN FILTER ( tblOne_1, [Value] = one )
    )
VAR tblthree_2 =
    GENERATE (
        tblTwo_2,
        VAR one = [@candidate] RETURN FILTER ( tblOne_2, [Value] = one )
    )
VAR ternary_1 =
    COUNTX ( tblthree_1, [@candidate] )
VAR ternary_2 =
    COUNTX ( tblthree_2, [@candidate] )
VAR ternary =
    IF ( ternary_1 <> BLANK () || ternary_2 <> BLANK (), 1, 0 )
RETURN
    ternary

simply put , GENIOUS! thank you very much, I had that on my mind but wasn't able to translate it to DAX. thank you very much!