E11000重复键错误，带有明确的_id分配，声称它为null，但实际上不是

It's the old E11000 error but with a twist (and yes I searched and found many of the same question but without the exact details that apply here - mainly the explicit _id assignment).

I'm manually assigning the _id field with bson.NewObjectId() because I need to return the ID back to the client - this was based on discussions from SO and GitHub, for example: https://github.com/go-mgo/mgo/issues/453

https://github.com/Southclaws/ScavengeSurviveCore/blob/master/storage/player.go#L28-L32

The ID field is set up in the struct as:

ID bson.ObjectId `json:"_id" bson:"_id"`

https://github.com/Southclaws/ScavengeSurviveCore/blob/master/types/player.go#L9-L23

So far I don't think I've missed anything. Unit tests create 3 instances of this object one after the other:

https://github.com/Southclaws/ScavengeSurviveCore/blob/master/server/handler_admin_test.go#L21-L56

First one gets created fine, it has an _id assigned to it (checked via mongo-express) and I even printed out the ID right before .Insert was called, same ID, everything is fine:

CREATING {ObjectIdHex("5b092f4aff3055468e3ce00a") playerID1 (...)

Problem arises when the next of the three test records gets inserted - exact same code, and the log is printed:

CREATING {ObjectIdHex("5b092f4aff3055468e3ce00b") playerID2 (...)

but it returns an E11000:

E11000 duplicate key error collection: <db>.<collection> index: <collection>_id_1 dup key: { : null }

From what I can gather, it thinks _id is null but given all the evidence I can see, it's not null at all because bson.NewObjectId() is assigned to the _id field right before .Insert is called - and the object is printed out right before insertion and the ID is there.

What am I missing here?