This patch upgrades version 1.0 to 1.1, so before you download and install it, make sure you do not already have version 1.1. You will find the version number in the bottom right corner of the main menu screen.
In addition to fixing many minor script bugs, this patch fixes the following problems:
This patch will not invalidate your saved games, however any saved game which has already been corrupted will not be fixed by installing this patch. For example, if you have a saved game which contains the duplicate-NPC bug, loading that saved game after installing this patch will not remove the duplicate NPC's from your party.
These patches are for the US and UK versions only. As far as I know, the German and French version were 1.1 to begin with and thus don't need a patch.