Concurrency issue while accessing Dbcontext in EF Core

I hooked up an Eventhandler to RabbitMQ message publisher and I am trying to insert those messages into a DB table as and when they are published. But when multiple messages are published simultaneously I am facing issue to access the database while querying. A System.NotSupportedException is thrown saying that a second operation has started before the previous asynchronous operation is completed.

public class RmqMessageHandler
{

    public void Register()
    {
        EventingBasicConsumer consumer = new EventingBasicConsumer(RmqChannel);
        consumer.Received += EventingBasicConsumer.Received;
        RmqChannel.BasicConsume(queue: QueueName, autoAck: false, consumer: consumer);

    }

    private async void EventingBasicConsuner_Received(object sender, BasicDeliverEventArgs message)
    {
        var body = message.Body;
        var encodedmessage = Encoding.UTF8.GetString(body.Span);
        var transaction = await DbRepo.Query<AccTransaction>().Where(X => X.Id == encodedmessage.Id).ToListAsync();
        if(transaction.count == 0)
        {
            DbRepo.StageNewItem(transaction);                
        }
    }
}

I have tried using lock to lock the dbcontext in order to avoid concurrency issue like below.

lock(locker)
{
    var transaction = DbRepo.Query<AccTransaction>().Where(X => X.Id == encodedmessage.Id).ToList();
    if(transaction.count == 0)
    {
        DbRepo.StageNewItem(transaction);                
    }
}

but that is also throwing System.Data.Entity.Core.EntityException saying The underlying provider failed on open or System.InvalidOperationException saying that the previous connection was not closed.

How can I overcome this issue..