The outcome of a Clan War is determined by which clan manages to get the most stars during the war, or on total destruction percentage if Stars are tied. If more than one player raids a given base, only the strongest attack will be counted toward your Clan’s final score. This means that if you're thinking about attacking a base that has already been 2-starred, it will only have one star left for the taking (and that’s only if you completely wipe it out).
Similarly if one of your clan members has already 3-starred an opponent’s war base, attacking the same base again will not increase your chances of winning the war.
Winning at Clan Wars requires planning and tactical resolve. It’s up to you to decide how to make the most of your clan member's strengths. Chat with your clan mates, pinpoint optimal opponents, and make each of your attacks count. The game will help you find a suitable opponent by offering recommended targets for everyone. However, its up to the Clan members to decide who attacks which village.
Every clan member who deals at least some damage during a Clan War, whether they score a Star or not will earn some bonus loot. However, only the winning clan will receive the full War Win Bonus.