Sharing excel file on LAN

I have an excel file with a userform which is used to input data.

Whenever the userform is initialized, a sr No(In a textbox) is automatically generated based on the max value in column A.The user then enter other data as per the fields in the userform.

When the submit(Command button) is pressed the data is enter in an excel sheet of the same workbook.

I want to share this workbook over my LAN and allow different users to edit at the same time.

The problem is when two users are accessing the workbook at the same time,the Sr No generated is same for both of them.

Now when user 1 saves the file the data is recorded in the worksheet, but when user 2 saves the file, the data gets overwritten and only what user 2 has recorded gets saved.

How to overcome this problem.

Workbook Attached.