removing duplicated rows without losing information

hello, George_Gian aggregatedColumns argument looks like this:

{
  {"column_name_01", aggregation_function_01},
  {"column_name_02", aggregation_function_02},
  ...,
  {"column_name_N", aggregation_function_N}
}

So it's a list of lists. Each nested list consists of column name and function (or aggregation function). Table.Group passes single argument to this function - the table made by original table grouping. Aggregated function must do something (or nothing - up to you) to this table and land the result into "column name" column. Try to create new columns yourself using different table functions like Table.Skip, Table.RemoveFirstN, Table.Max etc. or your own custom functions.

As mentioned earlier, aggregation function is always a function of single argument (table). So it must be defined as 

(x) => ... or (any_other_single_variable_name) => ... or (_) => ... or each ... (which is equivalent of (_) => ...). If applied library table function or your own custom function is a function of single argument then explicit variable definition may be ommited like this: {"row count", Table.RowCount}

Read more about Table.Group arguments on Rick de Groot's powerquery.how web site.