More than 1 user inserting data in SQL from ASP.net same time

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ASP.NET

SQL-Server

I am working on an ERP based project which was created using asp.net(3.0)c# as the front end and sql server(2005) as the back end. We have completed the project but after installation on the client location, when Multiple users Login and use the same form to perform a transaction, then the data for all users is mismatched. While we have Created Seperate userid and Login for all user. Can anybody suggest to me what I should do?


I have defined Windows authentication mode
On login form i am checking for user from the database whether the password and username is right or not.
1.SP_1 is executed to insert the data in table one
2 the primary key from table is retrived to aspx.cs page.
3 That primary key is provided to SP_2 along with other parameters to insert into table two.

suppose user one entered the data in header section (for table 1) and added two or more details in detail section (for table 2).using hid id
Also the user two entered the data in header section (for table 1) and added two or more details in detail section (for table 2).using his id
Both they press the save button at the same time
in this some time data of one user is not saving .
but if saved then it display the user two data to user one and vice versa.

Please Help.