ASP.NET Core Custom validation of an action parameter that is a list

Validation.cs

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc(options =>
        {
            options.ModelValidatorProviders.Insert(0, new ListOfFooValidatorProvider());
        });
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseMvc();
    }
}

public class ListOfFooValidatorProvider : IModelValidatorProvider
{
    public void CreateValidators(ModelValidatorProviderContext context)
    {
        if (typeof(IList<Foo>).IsAssignableFrom(context.ModelMetadata.ModelType))
        {
            context.Results.Add(new ValidatorItem()
            {
                Validator = new ListOfFooValidator(),
                IsReusable = true
            });
        }
    }
}

public class ListOfFooValidator : IModelValidator
{
    public IEnumerable<ModelValidationResult> Validate(ModelValidationContext context)
    {
        if (!(context.Model is IList<Foo> foos)) { yield break; }

        var hasDups = foos.GroupBy(f => f.Value).Any(g => g.Count() > 1);
        if (hasDups)
        {
            yield return new ModelValidationResult(null, $"Duplicate {nameof(Foo.Value)}.{nameof(Foo.Value)} detected");
        }
    }
}

[Route("api/[controller]")]
public class FooController : Controller
{
    [HttpPost]
    public IActionResult Post([FromBody] IList<Foo> foos)
    {
        return !ModelState.IsValid
            ? (IActionResult) BadRequest(ModelState.Values.SelectMany(x => x.Errors.Select(e => e.ErrorMessage)))
            : Ok();
    }
}

public class Foo
{
    public string Value { get; set; }
}