When we want to insert data in an array in Mongo we use $push . But it won't prevent you from inserting multiple times the same value in your array.
If you want to ensure that there's no duplication.
You can use : $addToSet
How it works
When you want to insert an element inside an array, $addToSet will check if this element already exists and if that's the case, will keep the existing one without pushing the element (but it will update your updatedAt field).
If your array doesn't exist yet, it will create and push the element inside it.
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$addToSet work only on array field