[{"Sigle_1":"BIOS","Sigle_2":"BFTA"},{"Sigle_1":"BERP","Sigle_2":"BEST"},{"Sigle_1":"BHDL","Sigle_2":"BIOS"},{"Sigle_1":"BIPM","Sigle_2":"BEST"},{"Sigle_1":"BHDL","Sigle_2":"BIOS"}]

db.collection.aggregate([{$facet:{array1:[{$group:{_id:"$Sigle_1"}}],array2:[{$group:{_id:"$Sigle_2"}}]}},{"$addFields":{"data":{"$concatArrays":["$array1","$array2"]}}},{"$unwind":"$data"},{"$replaceRoot":{"newRoot":"$data"}},{"$group":{"_id":"$_id"}},{$project:{Sigle:"$_id",_id:0}}])