Double Narwal Tusks (see the image in the header at the top of this page) are very rare and highly prized by hunters and collectors. They are also known as Twin Narwhal Tusks.
Normally, a narwhal tusk appears only in the male narwhal, and grows from the left tooth in the upper jaw. When double narwhal tusks develop, the right tooth has also grown into a tusk.
Here are some links related to double narwhal tusks:
Young Inuk Hunter Bags Rare Trophy
An Unusual Unicorn