Handling Multiple Users saving record at smae Time - Handling Unique Keys

Hi all,

I have a situation.

I have  a property Master entity.i have to assign a property to a User.

Eg: User Name : ROBIN

Property Name :VGN D Block

When user tryto save the record am enforcing to check whether the property is still available.If the property is still available then it will allow user to save the record.

But there is a scenario, when 2 users login at same time in 2 different pc..and when they try to assign a property for 2 users

Eg: USerNAMe1 : JASON

Property NAME :VGN D Block

Eg: User Name2 : ROBIN

Property Name :VGN D Block

And when user click save on same moment....property is assigned to 2 users..

How to implemet Primary key feature in CRM?