Try the following for a case-insensitive query against c:\temp\example.txt (assuming your SQL server is case-insensitive):

 

SELECT 

    COUNT(*)-1 AS Duplications,

    TextLine

FROM

(

    SELECT

        F1 AS TextLine

    FROM OpenDataSource( 

        'Microsoft.Jet.OLEDB.4.0',

        'Data Source="C:\TEMP\";Extended properties="Text;HDR=NO"')...Example#Txt

) TextInfo

GROUP BY TextLine

HAVING COUNT(*) > 1

COMPUTE SUM(COUNT(*)-1)

Or, for a case-sensitive query, try:

 

SELECT 

    COUNT(*)-1 AS Duplications,

    TextLine,

    Checksum

FROM

(

    SELECT

        F1 AS TextLine,

        BINARY_CHECKSUM (*) AS Checksum

    FROM OpenDataSource( 

        'Microsoft.Jet.OLEDB.4.0',

        'Data Source="C:\TEMP\";Extended properties="Text;HDR=NO"')...Example#Txt

) TextInfo

GROUP BY TextLine, Checksum

HAVING COUNT(*) > 1

COMPUTE SUM(COUNT(*)-1)

For my testing, c:\temp\example.txt contained:

This line appears twice (with different case)