Removing most recent dates of duplicate rows.

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test (later on when you use the query on your dataset, you will have to change the source appropriately. If you have columns other than these, then delete Changed type step and do a Changed type for complete table from UI again)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45Wck1Ly0zOTM0r8UlMKlbSUTLUNzLSNzIwtFAwsjK0AAo4GhoZAylvbSDhX5SSWqQQlJqcmlmWmqIUq4NPv4mVkTlEvwmQCsnzJNUAUytDEyoZgNMHsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Lab Name" = _t, #"Date/Time" = _t, SID = _t, Test = _t, Action = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Lab Name", type text}, {"Date/Time", type datetime}, {"SID", type text}, {"Test", type text}, {"Action", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Lab Name", "SID", "Test", "Action"}, {{"Date/Time", each List.Min([#"Date/Time"]), type nullable datetime}}),
    #"Reordered Columns" = Table.ReorderColumns(#"Grouped Rows",{"Lab Name", "Date/Time", "SID", "Test", "Action"})
in
    #"Reordered Columns"

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Vijay_A_Verma  Thank you so much! I studied your example and now clearly understand what I was doing wrong! Thank you! Thank you! Thank you!