No mention of what happens if we attach with an entity ID that is already tracked?

### Type of issue

Missing information

### Description

Greetings.

I couldn't find any explicit mention of this scenario:

Let's say an entity *E1* with primary key *K* is attached (no matter how, no matter what *K* is and no matter *E1*'s [EntityState](https://learn.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.entitystate?view=efcore-9.0)).

Then, **another** entity *E2* that **has the same** primary key *K* is attached with a call to `Attach(TEntity)` or `Attach(Object)`).

If I extrapolate what's described in the documentation:
- *E2* should end up being attached.
- *E2*'s EntityState will be *Added* if *K* is the CLR default for the property type, or *Unchanged* if *K* is something else (see [here](https://learn.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.attach?view=efcore-9.0)).

But what happens to the ChangeTracker's reference to E1? I guess it is replaced with one to E2? I think it would be worth explicitly stating what happens in this case.

Note that I couldn't find any information about this in these pages (it's possible I missed it though!):

- https://learn.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.attach?view=efcore-9.0
- https://learn.microsoft.com/en-us/ef/core/change-tracking/
- https://learn.microsoft.com/en-us/ef/core/change-tracking/explicit-tracking#attaching-existing-entities

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.attach?view=efcore-9.0

### Content source URL

https://github.com/dotnet/EntityFramework.ApiDocs/blob/live/dotnet/xml/Microsoft.EntityFrameworkCore/DbContext.xml

### Document Version Independent Id

db1657b7-d390-7a27-062c-408e166a85ca

### Platform Id

5236a0ac-0a7a-2052-3a45-24d4543bb1b7

### Article author

@dotnet-bot