Remove duplicates by prioritizing rows based on another column

Alright, the solution I came up with is a bit more complicated than it probably needed to be but it's all I got. Here is what I recommend:

Duplicate the LHM column. In the Transform section, divide column by Number of Characters (position 26) and by specific character (.). This will leave you with two columns, one with a 4 digit number (3422 for example) and a second column with the word pdf for the separator. Delete the pdf column. 

Now, duplicate the NVE column. 

Now, join the duplicate LHM column with the 4 digits and the NVE duplicate column. Since every NVE is unique, the only duplicate values in this new column will be when the 4 digits from the LHM are identical AND when the NVE column was blank, therefore not adding anything to the values.

Finally, click on the joined column from both duplicates and erase duplicates. From here, you can delete the joined column since the functions work without it. There you have it, you'll have a data set with duplicates but only when NVE is not empty. 

Hi - no problem at all.  You don't need an index column.  That just happened to be the column in my sample data that I wanted to use as the column from which duplicates should be removed.  See this new script below.  I have added comments and examples of what you need to change to make it work for you.  Please let me know if you have any other questions.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXJ0cgaSSrE6yFzfxJKMYrCYEZDn5OwCJF3z0nMyizPQRMFcYyDD2cUVRSNCDMw1ATJcXN1gXFOQgW7uyMbGAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Student ID" = _t, #"Student Name" = _t, Subject = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Student ID", Int64.Type}, {"Student Name", type text},  {"Subject", type text}}),
    #"Grouped Rows" = Table.Group(
        #"Changed Type", 
        // Column(s) containing the values from which you'd like duplicates removed.  
        // Replace this with the NVE numbers column. {"NVE-Nr."}
        {"Student ID", "Student Name"}, 
        {
            // Column that contains blank/empty/null values that should be removed if 
            // another row exists that is not blank/empty/null.
            // "Subject" will be the name of the new column.
            // _[Subject] is the column in the current table whose values should be evaluated.
            // Your column name in the current table will need to be referenced a little differently
            // since it contains a hyphen.
            // { "LHM-Nr.", each List.First ( List.Sort ( _[#"LHM-Nr."], Order.Descending ) ) },            
            { "Subject", each List.First ( List.Sort ( _[Subject], Order.Descending ) ) },
            { "All", each _, type table }}
    )
in
    #"Grouped Rows"

Hello again - here is the same query as the last response with your source data plugged in.  Note - I am not able to download your files (blocker on my end), so I converted the image of your original table to an actual table.  

The only difference between the last query and this one is the value of the Source step.  Everything else is the same.  In case you are not already doing so, I recommend you copy this script, paste it into a blank query and then view the output.

BEFORE:

AFTER:

SCRIPT:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("vZVPa4MwGIe/i+ci778k5ihsDEYZwjZ2KEUctWPQw2D7/uxN5oyhxZMaQjAa8jzmF+PhUBAQAoK04MWRb/fdqb98fv/0LQtR+XU6F7sCpAQsw1DtvNUobAS8XnuPxGIgFATnOdzU2tR7bcPopvvo9VnsHXdL84imPLcgz4C5wYNqyqtW56GZ8vzq6xnz0wm1OIfWpijD2r5075cBDmuFOcB1pd0IJ14SPpNsevMJXNaHx5gTvEpwMwtHRgsuh5tb8Pr1+QGB2WHoPT7dGUBk0UmAtIrlv31239TaMs9ttOWglEFlGyhmULMNFDKo3QbK46YiQrSJP/8tL5vvuKtjvoNAtZEAjgJ6ssWsBwG/kQAkAWsoCQjMCNgg4BEzAbH/AvZaYDi+rgWcHQWAIVIHgfzfrPzjLw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Ladeliste = _t, Datum = _t, #"Auftragsnr." = _t, #"NVE-Nr." = _t, #"LNM-Nr." = _t, #"LHM-Typ" = _t, Index = _t, Page001 = _t, DeleteRowIndex = _t]),
    grouped = Table.Group(
        Source, 
        // Column(s) containing the values from which you'd like duplicates removed.  
        {"NVE-Nr."},
        {
            { "Table", // Name of the new column.
                each 
                //---------------------------------------------------------
                // Option 1: Sort descending, then select the first result.
                //---------------------------------------------------------
                // This will work if there is a maximum of only two rows per NVE-Nr.
                //let varTable = Table.Sort ( _, {{"LHM-Nr.", Order.Descending}}) in
                //Table.FirstN ( varTable, 1 ),
                //---------------------------------------------------------
                // Option 2: Select the last LHM-Hr for each NVE-Nr.
                //---------------------------------------------------------
                // This will work if the row to keep always appears last in the group.
                Table.LastN ( _, 1 ),                
                type table
            }
        }
    ),
    expand = Table.ExpandTableColumn ( 
        grouped, 
        "Table",                                  // Expand the tables in this column
        List.Difference (                         // New column names
            Table.ColumnNames (                   // are the column names 
                Table.Combine ( grouped[Table] )  // in the nested tables 
            ), 
            Table.ColumnNames ( grouped )         // that do not appear in the grouped table.
        ) 
    )

in
    expand