Well, this is up to you, since you're giving away this bundle. If someone wins the bundle who owns some of the games already you can ask for a reroll. So it's up to you to define what's an acceptable amount of pre-owned games from this pack.
Anyway, it's a great way to start giving away =) Welcome to this community!

As Klinge says, it depends on you. Some folks do not want to give a bundle away to someone who owns many of the games, since they feel it's a waste. It's entirely up to the giveaway creator to put such limitations and/or ask for a reroll. I will enter too. Have 2 of the games. Just ask for a reroll if you see that the winner owns too many and you have an issue with that. :) And thank you for one amazing bundle! Great start on the site!