added `ArgumentExcpetion` if `validates` has duplicated attributes.

Before this PR you can pass the same attribute to the same `valitates`
method.

Example:

    Person.validates_presence_of :name, :name
    person = Person.new

    # before
    person.valid?
    person.errors.size #=> 2

    # with this pr
    person.valid? #=> `ArgumentError Exception`

test