Whenever I am trying to open eclipse, two instances of it pointing to the same workspace is opened every time. When I close one, both of them get closed.
Is it some feature I accidentally enabled? How to revert it back ?
Perhaps you have Clicked on Window/New Window? That would open a second eclipse window with the same workspace. Normally a workspace cannot be open in two different eclipse instances.
When you try to close one of the windows using the close-button in the window decoration, you should be able to get back to normal one-window-mode.