Welcome to Librarium Online!
A buddy of mine plays eldar and says:" look, in these entries it says 2 of the same weapons count as twin linked, and in these it doesn't say that, so it doesn't count as twin linked".
Does it need to be typed in the entry or does it automatically count as twin linked if you take 2 of the same weapons? Because the warwalkers (i think) with non linked scatter lasers (i think) HURT, and are rediculously cheep (for walkers) for instance
also how does the avatars melta sword work?
Your buddy is right: War Walker weapons are never twin linked.
As to the second point, the avatar is a MC, so it hits with 2d6 armour pen in melee.
For the Melta part, it's used just like a melta weapon at the range. 12 inches, I believe, so when thrown within 6, the attack gains 2d6 armour pen