i already have most and i'm saving points so i will not join for now but i think that you can always join and if he doesn't agree he will just reroll

I'd probably ask people with at least 3 titles to refrain from joining.
Since it's a 6 title pack and it's a private giveaway with $100 CV requirement, i think it's ok for people with up to 2 titles to join.

More because of this... Even though it's one of the "old" type of gifts, dunno if new steam policy applies. I had for instance Castle Crashers, then got CSL bundle, tried to activate it, it warned me that it WOULDN'T give me an extra copy of Castle. :/
So if my hunch is correct, people joining with 3 out of the 6 games, those 3 will go to waste if one of those is the winner. Which is kinda sad imo. Let's hope it's not, which will enable the winner (if someone with any of the titles) to pay it forward if they want by gifting what they already had on another giveaway (no strings attached - just feel it would be the nice thing to do).

So, TL;DR; short answer: Yes, it's ok for both of you and everyone with less than 3 out of 6.