Problem With Distributed Transactions - Multiple Threads Pop The Same Message From Queue

Hi,

I am using distributed transactions where in I start a TransactionScope in BLL and receive data from service broker queue in DAL, perform various actions in BLL and DAL and if everything is ok call TransactionScope.Commit().

I have a problem where in if i run multiple instances of the same app ( each app creates one thread ), the threads pop out the same message and I get a deadlock upon commit.

My dequeue SP is as follows:

CREATE PROC [dbo].[queue_dequeue]
@entryId int OUTPUT
AS
BEGIN
DECLARE @conversationHandle UNIQUEIDENTIFIER;
DECLARE @messageTypeName SYSNAME;
DECLARE @conversationGroupId UNIQUEIDENTIFIER;

GET CONVERSATION GROUP @conversationGroupId FROM ProcessingQueue;
if (@conversationGroupId is not null)
BEGIN
RECEIVE TOP(1) @entryId = CONVERT(INT, [message_body]), @conversationHandle = [conversation_handle], @messageTypeName = [message_type_name] FROM ProcessingQueue WHERE conversation_group_id=@conversationGroupId
END

if @messageTypeName in
(
'http://schemas.microsoft.com/SQL/ServiceBroker/EndDialog',
'http://schemas.microsoft.com/SQL/ServiceBroker/Error'
)
begin
end conversation @conversationHandle;
end
END

Can anyone explain to me why the threads are able to pop the same message ? I thought service broker made sure this cannot happen?