Simple web application with ASP.NET MVC + EF Core sometimes creates several identical entries in the database

I have a web application where users can post offers for products.

Sometimes - less than 1 in 10 cases - saving an offer leads to multiple entries in the database with identical data (usually 2 entries, but 3 or even 4 identical entries already occurred).

The offer class is a simple entity class with some properties in it. It is also connected to two further entity classes OfferImage and OfferCategory, which store the associated images and the categories in which the offer should appear.

The code to save an item to the database is the following (part of a repository class):

public class OfferRepository {
    ...
    public async Task InsertAsync(Offer offer)
    {
        Context.Offer.Add(offer);
        await Context.SaveChangesAsync();
    }
    ...
}

It is called within a service class:

public class OfferService 
{
    ...
    public async Task CheckinAsync(Offer offer)
    {
        await repository.InsertAsync(offer);
    }
    ...
}

That method of this service class is called by an mvc controller:

public async Task<IActionResult> Create(CreateOfferViewModel createOfferViewModel)
{
    if (ModelState.IsValid)
    {
        ...
        //conversion of the view model object to an Offer object
        ...
        await offerService.CheckinAsync(offer);
    }
    ...
}

As you can see, the structure is relatively simple. However, this error occurs regularly.

The lifecycle of the service classes is managed with dependency injection in startup.cs. OfferRepository and OfferService are both added scoped (services.AddScoped) The context class is added like this:

services.AddDbContext<Context>(options =>
{
    options.UseSqlServer(
        Configuration.GetConnectionString("DataConnection"),
        sqlServerOptionsAction: sqlServerOptions =>
            {
                sqlServerOptions.EnableRetryOnFailure(maxRetryCount: 3, maxRetryDelay: TimeSpan.FromSeconds(3), errorNumbersToAdd: null);
            }
    );
});

To further narrow down the problem, I ran a profiler and got a recording of the INSERT statements (in chronological order):

1. SPID 59 - 2019-09-24 16:05:19.670 - INSERT INTO Offer ...
2. SPID 57 - 2019-09-24 16:05:19.673 - INSERT INTO Offer ...
3. SPID 59 - 2019-09-24 16:05:19.710 - INSERT INTO OfferImage ... / INSERT INTO OfferCategory ...
4. SPID 57 - 2019-09-24 16:05:19.760 - INSERT INTO OfferImage ... / INSERT INTO OfferCategory ...

What makes me suspicious is that there are two different process IDs that execute the INSERTs. Since the DbContext is scoped by default, there should only be one process ID under which all statemens are executed - or am I wrong? If I am not mistaken, this would mean that two requests are executed in parallel, which in turn raises further questions about how this can happen.

As you can see, I am a little confused and hope for help from someone who can explain this or has observed and solved something similar.

(SQL Server is version 13/2016, EF Core is version 2.2)

Thank you very much!