In the month of January 2014 the creators of Ninja Warz made all relics half off for a limited time. They did this by removing all relics and re-adding them at half-price.
This exploit was done by buying a relic during the half-off sale that you already owned. The game thinks it's a different relic due to it's seperate ID. Therefore you can have on two of any relic you want if you purchased a duplicate of it during the sale.