How do i open a document in the existing window instead of it opening a new window?

if i start writer, it opens a blank document. if i then use the menu to open an existing document, it opens it in a new window, not the one with the blank document, which i then have to close. how do i get it to re-use the existing window instead.

I tested in LibreOffice on Windows XP. Start Writer and new document is opened. Open existing document and “new document” window is automatically closed.

What is the version of LibreOffice you are using from Help | About menu? What is your operating system name/version (hard to give detailed instruction if this info is missing)?

Did you change some settings in LibreOffice. Maybe you need to delete LibreOffice profile (make a backup before deleting) to see if this is your profile problem.