mongo.IsDuplicateKeyError is a convenient helper to detect duplicates. It however does not work when the duplicate key error is returned as part of a bulk. While it will tell whether any of the errors in the bulk was a duplicate, it can't be applied to the individual errors (of type BulkWriteError) within the BulkWriteException unless I missed a conversion somewhere.

_, err := collection.BulkWrite(ctx, ops, options.BulkWrite().SetOrdered(false))
if err != nil {
    berr := &mongo.BulkWriteException{}
    if !errors.As(err, berr) {
        return err
    }    
    for _, we := range berr.WriteErrors {
        // >>>> 
        // >>>> Does not work: 
        // >>>> mongo.IsDuplicateKeyError() does not understand either 'we' (BulkWriteError) or 'we.WriteError' (WriteError)
        // >>>>
        if mongo.IsDuplicateKeyError(we) {
            continue
        }
        return err
    }
    // The bulk only contains duplicate key errors: all good
} 

It's a bit limiting since it's hard to decide what do with a whole batch just knowing that at least one of the operations failed due to a duplicate key error. Iterating on the errors in the bulk should allow mongo.IsDuplicateKeyError to be applied to them and return true if relevant. It has a code and a message so the same logic as for serverError should be applicable.