bulk_update() doesn't necessarily ignore duplicates.

[Tim McCurrach]

In the ​docs for bulk_update it says: "If objs contains duplicates, only the first one is updated." This is due to the way the query is constructed.

However this is only true for each SQL update. And if there are duplicates that fall into different batches, they will both (or all) be used as part of the update.

To Reproduce Error

>>> m1 = MyModel.objects.get(id=1)
>>> m2 = MyModel.objects.get(id=1)
>>> m1.name="a"
>>> m2.name="b"
>>> MyModel.objects.bulk_update([m1, m2], ['name'], batch_size=1)
>>> MyModel.objects.get(id=1).name
'b' 

Whilst the above is an extreme example, it demonstrates the point. If a large number of objects are being updated, you cannot currently rely on the behaviour that only the first instance of a duplicate will affect the update.

[Mariusz Felisiak]

I don't think it is worth additional complexity (see #29968), we can clarify this in docs, e.g.

If ``objs`` contains duplicates in a batch, only the first one is updated.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 241da3f0:

Fixed #32388 -- Clarified QuerySet.bulk_update() caveat about duplicates for multiple batches.

[Mariusz Felisiak <felisiak.mariusz@…>]

In e95a60b:

[3.2.x] Fixed #32388 -- Clarified QuerySet.bulk_update() caveat about duplicates for multiple batches.

Backport of 241da3f06ee0c6f436341cda5890b221ac453e3b from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In b920945d:

[3.1.x] Fixed #32388 -- Clarified QuerySet.bulk_update() caveat about duplicates for multiple batches.

Backport of 241da3f06ee0c6f436341cda5890b221ac453e3b from master