Mensagem:

"System.NotSupportedException was unhandled
Message: An unhandled exception of type 'System.NotSupportedException' occurred in mscorlib.dll
Additional information: A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe."

Código:

public async Task<IEnumerable<UserLangDTO>> ImportLang(int userId)
{
    var userLangs = new List<UserLangDTO>();
    using (FirstContext ctx = new FirstContext())
    {
        if (await (ctx.UserLang.AnyAsync(u => u.UserId == userId)) == false)
            //some exception here

        userLangs = await ctx.UserLang.AsNoTracking()
                                .Where(ul => ul.UserId == userId)
                                .Join(ctx.Language,
                                    u => u.LangID,
                                    l => l.LangID,
                                    (u, l) => new { u, l })
                                .Join(ctx.Level,
                                    ul => ul.u.LevelID,
                                    le => le.LevelID,
                                    (ul, le) => new { ul, le })
                                .Select(r => new UserLangDTO
                                {
                                UserId = r.ul.u.UserId,
                                Language = r.ul.l.Language,
                                Level = r.le.Level,
                                }).ToListAsync().ConfigureAwait(false);

    }
    using (SecondContext ctx = new SecondContext())
    {
        if ( await (ctx.UserLangs.AnyAsync(u => u.UserId == userId)) == true && userLangs.Any())
            ctx.UserLangs.RemoveRange(ctx.UserLangs.Where(u => u.UserId == userId));
        if (await hasUserLangs && userLangs.Any())
        {
            userLangs.ForEach(async l =>
            {
                var userLanguage = new UserLang();
                userLanguage.UserId = userId;
                userLanguage.LanguageId = await ctx.Languages.AsNoTracking()
                                                 .Where(la => la.NameEn == l.Language)
                                                 .Select(la => la.Id).FirstOrDefaultAsync().ConfigureAwait(false);
                userLanguage.LevelId = await ctx.Levels.AsNoTracking()
                                                .Where(la => la.NameEn == l.Language)
                                                .Select(la => la.Id).FirstOrDefaultAsync().ConfigureAwait(false);

                ctx.UserLangs.Add(userLanguage);
            });
        }
        await ctx.SaveChangesAsync().ConfigureAwait(false);
    }
    return userLangs;
}

O que eu tentei:

Não tenho certeza do que estou fazendo de errado, tentei coisas diferentes como:

1

await Task.Run(() => Parallel.ForEach(strings, s =>
{
    DoSomething(s);
})); 

2)

var tasks = userLangs.Select(async l =>
{
    //rest of the code here
}
await Task.WhenAll(tasks); 

3)

var tasks = userLangs.Select(asyn,c l =>
{
    //rest of the code here
}
await Task.WhenAll(tasks);

await ctx.SaveChangesAsync().ConfigureAwait(false); 

Outras tentativas de tentativa e erro que não lembro agora

O que estou fazendo errado?