Find mongo records with duplicated values in array field - arrays

Question

Consider this mongo collection:
[{
"_id": {
"s": "HU",
"k": 1
},
"boxed": {
"values": [{
"s": "NL",
"k": 2
},
{
"s": "BR",
"k": 3
},
{
"s": "NL",
"k": 2
}
]
}
},
{
"_id": {
"s": "FR",
"k": 2
},
"boxed": {
"values": [{
"s": "SE",
"k": 99
}]
}
},
{
"_id": {
"s": "UA",
"k": 14
},
"boxed": {}
}
]
I'm basically trying to find the records that have duplicated boxed.values. One such example would be the first one, where NL*2 is repeated twice.
My first idea was to project the original size of the values array, use $map to turn that array of objects into an array of strings (such as $map: { input: "$boxed.values", in: { $concat: ["$$this.s", "*", "$$this.k"] } }) and then remove the duplicates out of the array of strings, so i can compare the original size with the dupe-removed one. If the size would differ, then it would mean that record has duplicates.
However, it seems that there's no way in mongo (or at least I have not found one) to easily remove duplicated values out of an array of strings.
Any ideas?

Answer 1

You can do something like this,
[
{
$unwind: "$boxed.values"
},
{
$group: {
_id: "$_id",
"values": {
$addToSet: "$boxed.values"
}
}
},
{
$addFields: {
"boxed.values": "$values"
}
}
]
Working Mongo playground. $addToSet is an array operation which removes duplicates