You can do this, but you will not be able to compete in any of the scheduled tournaments. Having duplicate leaders will not cancel the bonus however, so this will not work.
If you are going to do this, why not just use 8 Kolos? Higher power and doesn't need Timber.
Well you can put them... But it wont work
The rules say "diferent cards from te same a clan" and Lenox is a single card. Also "cancel leader" works with ALL leaders.
Also you could only use that deck outside of tournaments. Please read the rules more carefully
Sure you can... just not in tournaments - it is not allowed (read the deck formats!).
It won't trigger the clan bonus, though.
However, leader clan "bonus" - i.e. ability-nullification, WILL trigger even with twice the same leader.
So, in summary: Sure you can put those in one deck but you will get neither the clan bonus for Lennox nor the +1 damage from Timber. Well, unless you get 3x Lennox, 1x Timber in which case you will get +1 damage on all cards.
In any case that deck will be very bad, likely beaten by most well-played 25 star decks and even if it wins it will likely get less points than the lower-star deck that lost against it.
Or in short: your idea does not work.
You can have as much cards as you want in a deck. On the other hand having more than one Leader in you hand would cancel each other's ability. So for example having two Timber at the same time won't give you +2 damage nor would it give you a +1 cause his ability would be canceled.