{
  "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"
}

{
  "Sigle": "BIOS",
},
{
  "Sigle": "BFTA",
},
{
  "Sigle": "BERP",
},
{
  "Sigle": "BEST",
},
{
  "Sigle": "BHDL",
},
{
  "Sigle": "BIPM",
}

db.users.aggregate([{$group: {_id: {Sigle1: '$Sigle_1', Sigle2: '$Sigle_2'}}}])

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 }}
])

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

{
  "Sigle_1": "BIOS",
  "Sigle_2": "BFTA",
  "Name_1": "Basic Input Output System",
  "Name_2": "Belgian Food Truck Association"

},
{
  "Sigle_1": "BERP",
  "Sigle_2": "BEST",
  "Name_1": "British Experimental Rotor Programme",
  "Name_2": "Board of European Students of Technology"
},
{
  "Sigle_1": "BHDL",
  "Sigle_2": "BIOS",
  "Name_1": "B Hardware Description Language",
  "Name_2": "Basic Input Output System"
},
{
  "Sigle_1": "BIPM",
  "Sigle_2": "BEST",
  "Name_1": "Brevet d'initiation au parachutisme militaire",
  "Name_2": "Board of European Students of Technology"
},
{
  "Sigle_1": "BHDL",
  "Sigle_2": "BIOS",
  "Name_1": "B Hardware Description Language",
  "Name_2": "Basic Input Output System"
}

db.collections.aggregate([
    { "$facet": {
        "array1": [
        { "$group": {
            "_id": "$Sigle_1"
            }}
        ],
        "array2": [
        { "$group": {
            "_id": "$Sigle_2"
            }}
        ],
        "array3": [
        { "$group": {
            "_id": "$Name_1"
            }}
        ],
        "array4": [
        { "$group": {
            "_id": "$Name_2"
                }}
            ]
        }},
        { "$addFields": {
            "data": {
            "$concatArrays": ["$array1", "$array2"]
            },
            "data_1": {"$concatArrays": ["$array3", "$array4"]
            }
        }}
])