Scratch on my Panasonic 20mm f/1. 7?
No, that's not normal. If there is indeed a scratch on the inside, it must have occured at the factory unless someone tried to disassemble it. It may also not actually be a scratch inside, but maybe a hair, thread, etc. got into the lens somehow. If its on the front element, it may be entirely invisible in most or all of your photos, but if it were me, I'd be picky and exchange it if I had the chance. If you ever attempt to resell it, blemishes will lower its resale value.