Accessing related fields with stored procedure

Hi
I have a problem I got a stored procedure to delete the rows of three related tables. But when i use the procedure without multiple occurances of the related fields i am successfully running this procedure....And when i create duplicate rows in the related table my procedure is giving error can any one correct my procedure and i have to take a field instead of ID that is when i call the delete procedure i have to take mapname instead of maptointernalid and mapfrominternalid which are related to MapID and inturn gives the MapName. Please anybody help I am so confused

My procedure as follows . The error occurs here (indicated with this sign "=>")

CREATE PROCEDURE SP_Delete_DocumentTypeInstance2
(
@MaptoInternalID int,
@MapfromInternalID int,
@DocumentTypeInstanceID int
)
AS

Set nocount on

Declare @MapID int


select @DocumentTypeInstanceID=(select DocumentTypeInstanceID from
OrganizationDocumentType where DocumentTypeInstanceID=@DocumentTypeInstanceID)

=> select @MapID=(select MaptoInternalID from DocumentTypeInstance where MaptoInternalID=@MaptoInternalID)

Delete from OrganizationDocumentType
where DocumentTypeInstanceID=@DocumentTypeInstanceID

Delete from DocumentTypeInstance
where MaptoInternalID=@MapID

Delete from Map
where MapID=@MapID

set nocount off
GO


thanks in advance
sai11