Entity Framework core failing on async methods

I'm using async methods with EF Core - getting the context through the built in DI

 public async Task<bool> Upvote(int userId, int articleId)
{
    var article = await context.Articles
                               .FirstOrDefaultAsync(x => x.Id == articleId);
    if (article == null)
    {
        return false;
    }

    var existing = await context.Votes
                                .FirstOrDefaultAsync(x => x.UserId == userId
                                                       && x.ArticleId == articleId);
    if (existing != null)
    ...

which is ran when someone upvotes an article.

Everything runs fine if this function gets ran one at a time (one after another).

When I hit this function several times at the same time, I get this exception:

fail: Microsoft.EntityFrameworkCore.Query.Internal.MySqlQueryCompilationContextFactory[1]
      An exception occurred in the database while iterating the results of a query.
      System.NullReferenceException: Object reference not set to an instance of an object.
         at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.<BufferAllAsync>d__12.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

The breakpoint hits: var existing = await context.Votes.FirstOrDefaultAsync(x => x.UserId == userId && x.ArticleId == articleId);

I'm also getting this error: Message [string]:"A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread safe."

What are some possible solutions?

Edit 1: This is how I'm setting up the context: In Startup.cs, I configure the context:

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ArticlesContext>(options => 
        options.UseMySql(Configuration.GetConnectionString("ArticlesDB")));
...

And then I inject it in the constructor of the containing class:

private ArticlesContext context;
private ILoggingApi loggingApi;

public VoteRepository(ArticlesContext context, ILoggingApi loggingApi)
{
    this.context = context;
    this.loggingApi = loggingApi;
}

Edit 2: I'm awaiting all the way down to the controller via:

public async Task<bool> Upvote(int articleId)
{
    return await this.votesRepository.Upvote(userId, articleId);
}

And then in the controller...

[HttpPost]
[Route("upvote")]
public async Task<IActionResult> Upvote([FromBody]int articleId)
{
    var success = await votesService.Upvote(articleId);
    return new ObjectResult(success);
}

Edit 3:

I've changed my services/repos to be transient instead of singletons, but now I'm running into another issue with :

public int getCurrentUserId()
{
    if (!httpContextAccessor.HttpContext.User.HasClaim(c => c.Type == "UserId"))
    {
        return -1;
    }

It's the same async issue - but this time, HttpContext is null. I'm injecting the context accessor via

public UserService(IUserRepository userRepository, IHttpContextAccessor httpContextAccessor)
{
    this.userRepository = userRepository;
    this.httpContextAccessor = httpContextAccessor;
}

Answer: IHttpContextAccessor needs to be registered as a singleton an not transient services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();