In various applications, a user is only expected to perform a certain action once, such as voting, requesting a refund, or making a purchase. When this restriction is not enforced, sometimes this can have security implications. For example, in a voting application, an attacker could attempt to "stuff the ballot box" by voting multiple times.

If these votes are counted separately, then the attacker could directly affect who wins the vote. This could have significant business impact depending on the purpose of the product.