Remove same occurrence if a value is found

Here's the M code that does it. Paste into Power Query's Advanced Editor and test it with your datasets.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUfLJT8/MA9KG+gaG+kYGRgYKhgZWBgZKsToQ+bDM1HI80i6pOaklqSTqNyRaOiCxMjc1rwRDhSFYhROS+43g8pYw652Q3YdDHmEBigJTpdhYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [User = _t, Txn = _t, DateTime = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"User", type text}, {"Txn", type text}, {"DateTime", type datetime}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Keep Row?", 
        each
            if Text.Lower([Txn]) = "login" then 
                true
            else
                Table.IsEmpty(
                    Table.SelectRows(
                        #"Changed Type",
                        (r) =>
                            r[User] = [User]
                            and r[DateTime] = [DateTime]
                            and Text.Lower(r[Txn]) = "login"
                    )
                )
    ),
    #"Changed Type1" = Table.TransformColumnTypes(#"Added Custom",{{"Keep Row?", type logical}}),
    #"Filtered Rows" = Table.SelectRows(#"Changed Type1", each ([#"Keep Row?"] = true)),
    #"Removed Columns" = Table.RemoveColumns(#"Filtered Rows",{"Keep Row?"})
in
    #"Removed Columns"

Best

D