Internet Explorer is a browser offered by Microsoft. It's included with the Windows operating system but is also available for free download from Microsoft. Internet Explorer saves copies of your tabs. If you have two browser windows open, and your computer crashes, you open two browser windows and the windows' tabs when you recover from the crash.
The more recent versions of Internet Explorer (versions 7, 9 and 9) provide you with tabs, which let you open multiple web pages within one browser window. Previous versions require you to open multiple browser windows. If your computer crashes and you restart Internet Explorer, the system restores your two windows, which causes two windows to open when you click the shortcut.
Some websites have ads programmed into the home page code. If you set this kind of website as your home page, you open the home page and then the website ad opens. This causes you to see two opened browser windows. To stop two windows from opening, you need an ad blocker or you must change your browser's home page.
Sometimes you open a second browser window just from accidentally clicking the icon twice. This happens if you have a shortcut on your Windows taskbar in Windows 7. The icon in the task bar acts as a quick link, and if you accidentally click it twice, you open two browser windows.
A virus or adware installed on your computer sometimes changes your browser's settings. A virus can change your browser's home page, and it then opens a second ad window. Antivirus software helps stop the installation of web browser hacks.