Each time you import an OST Takeoff or Project file into a database, the program creates another copy of the bid. Existing copies are not overwritten.
On-Screen Takeoff registers the "ost" extension as an On-Screen Takeoff project file, however, several other programs (most notably, Microsoft Outlook), may change that registration. If you double-click on an OST file and it doesn't open On-Screen Takeoff, right-click it, select "Open With" and set On-Screen Takeoff as the default file handler - see your Windows documentation for details.