How to run SpecFlow tests in parallel

Question

I try to run SpecFlow tests with NUnit test runner in parallel. I am using:

• C# /.Net Core 3.1
• NUnit as testrunner
• Specflow

I have added this line on top of a file (as describe here: https://specflow.org/documentation/parallel-execution/ ):

[assembly: Parallelizable(ParallelScope.All)]

Tests start to execute in parallel but immediately throw an error:

Message: 
    System.ArgumentException : An item with the same key has already been added. Key: NUnitTestProject1.SpecFlowFeature1Steps
Stack Trace: 
    Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
    Dictionary`2.Add(TKey key, TValue value)
    TypeRegistration.Resolve(ObjectContainer container, RegistrationKey keyToResolve, ResolutionList resolutionPath)
    ObjectContainer.ResolveObject(RegistrationKey keyToResolve, ResolutionList resolutionPath)
    ObjectContainer.Resolve(Type typeToResolve, ResolutionList resolutionPath, String name)
    ObjectContainer.Resolve(Type typeToResolve, String name)
    TestObjectResolver.ResolveBindingInstance(Type bindingType, IObjectContainer container)
    lambda_method(Closure , IContextManager , Int32 )
    BindingInvoker.InvokeBinding(IBinding binding, IContextManager contextManager, Object[] arguments, ITestTracer testTracer, TimeSpan& duration)
    TestExecutionEngine.ExecuteStepMatch(BindingMatch match, Object[] arguments)
    TestExecutionEngine.ExecuteStep(IContextManager contextManager, StepInstance stepInstance)
    TestExecutionEngine.OnAfterLastStep()
    TestRunner.CollectScenarioErrors()
    SpecFlowFeature1Feature.ScenarioCleanup()
    SpecFlowFeature1Feature.AddTwoNumbers() line 8

I tried without SpecFlow (just raw NUnit) and it worked. I also tried with MSTest test runner but got the same exception.

I have made a very tiny example project which you can clone here: https://github.com/davidguidali/specflowerror

Any help would be appreciated. :)

EDIT: Full code of steps

using NUnit.Framework;
using System;
using System.Threading;
using TechTalk.SpecFlow;

[assembly: Parallelizable(ParallelScope.All)]

namespace NUnitTestProject1
{
    [Binding]
    public class SpecFlowFeature1Steps
    {
        [Given(@"test(.*)")]
        public void GivenTest(int p0)
        {
            Thread.Sleep(5000);
            Assert.IsTrue(true);
        }
    }
}

Answer 1

I'm using this:

[assembly: Parallelizable(ParallelScope.Fixtures)]