CNContact instantMessage field duplicates after serializaiton/deserialization

Hello folks,

I stumbled upon a weird CNContact serialization problem. I use the Contacts framework to update the AIM field, which is one of the instantMessageAddresses within a single Contact. Here is the simplified code I used:

func updateAIMFieldOn(contact: CNContact, aimValue: String) {
    do {
        guard let mutableContact = contact.mutableCopy() as? CNMutableContact else {
            logger.error("[CM] Couldn't update contact with aim \(aimValue)")
            return
        }
        var updatedAddresses = mutableContact.instantMessageAddresses
        updatedAddresses.append(CNLabeledValue(label: "", value: CNInstantMessageAddress(username: aimValue, service: CNInstantMessageServiceAIM)))
        
        mutableContact.instantMessageAddresses = updatedAddresses
        
        let saveRequest = CNSaveRequest()
        saveRequest.update(mutableContact)
        
        try CNContactStore().execute(saveRequest)
        logger.verbose("Contact's AIM updated successfully!")
    } catch {
        logger.error("Couldn't update contact")
    }
}

And after serializing the contact to data, and then deserializing, the contact got two AIM fields with the same value:

X-AIM;type=pref:some:part:of_my_aim_value

IMPP;X-SERVICE-TYPE=AIM;type=pref:some:part:of_my_aim_value

Why does it work in this manner? Is it possible that ":" char causes that? Format of my aim username is {some:part:of_my_aim_value}. I didn't find any information in the docs.

Thanks!