I think the employee at your shop is misinformed. 1 kg of dextrose in 23 L of wort will add about 4°P/1.016 SG to your gravity, so nowhere near double. It is a cheap way to boost gravity and therefore ABV, and is used in some IPA sub-styles (West Coast in particular) as a matter of course. As far as the under-pitching, assuming it was a reasonable amount of yeast to begin with you should be just fine.