Duplicate or refer Table ? Or someting else

No!

You have to use a separate expression for 

Table.SelectColumns(Table.SelectRows(Table1, (sel)=> sel[#"filter_item"] = 1)),{"key_1", "field_1", "field_2", "field_3", "field_4", "field_5"})

for instance:

TABBUFF=Table.Buffer(Table.SelectColumns(Table.SelectRows(Table1, (sel)=> sel[#"filter_item"] = 1)),{"key_1", "field_1", "field_2", "field_3", "field_4", "field_5"}),

then the rest (check the sintax: ican't do)

#"join_sum_T1_T2_" = let #"GroupedLines" = Table.Group(TABBUFF, {"key", "filter_item"}, {{"sum_field_1", each List.Sum([#"field_1"]), type nullable number}, {"sum_field_2", each List.Sum([field_32), type nullable number}, {"Ssum_field_3", each List.Sum([#"field_3"]), type nullable number}, {"sum_field_4", each List.Sum([#"field_4"]), type nullable number}}), #"MergedQueries" = Table.NestedJoin(#"Table2_source", {"key_2"}, 
#"GroupedLines", {"key_1"}, "Table1_sum", JoinKind.LeftOuter), 
#"Tableexpanded" = Table.ExpandTableColumn(#"MergedQueries", "Table1_sum", {"sum_field_1", "sum_field_2", "sum_field_3", "sum_field_4"}, {"sum_field_1", "sum_field_2", "sum_field_3", "sum_field_4"}) 
in #"Tableexpanded",