Opening multiple windows at one call
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I am working on WPF Application. I have made small PointOfSale Type Application. When I sale the products the Bill/receipt window will open. 

It is working fine when I sale product 1st time or you can say whenever I run the application first time. But whenever I am going to sale products second time the Bill/receipt window will open 2 times, selling products third time the Bill/receipt window will open 3 times and so on...

I didn't get it how it happens. Any help can appreciate. 

Thank you in advance.