Yes, it is related to your active DLC's, and whether you had special,
importable gear equipped on the save file you imported from TW1. The behaviour of them reappearing magically, is specific to the Prologue. Each story part of the Prologue reloads your equipment. The behaviour will stop once you reach Chapter 1.
It's simple to deactivate DLC's that automatically add equipment to your inventory. The files you need to tamper with are all located in your installation directory;
The Witcher 2\Cooked PC. Look for the files
alchemy_suit.dzip;
magical_suit.dzip;
roche_jacket.dzip;
swordsman_suit.dzip. Each of these add, automatically, various powerful items to your inventory. Removing them from the folder is all you need to do in order to deactivate them.