Sign-up offer is limited to one per customer 1 comments

I have signed up for Adwords long time back and while signing up I have used a Promotional Code. Today I received a Promotional Code from Google. So I logged into my account to use the Promo code I received. While I used my code and clicked on Redeem, it displayed the message "Sign-up offer is limited to one per customer". It means that I could use promo code only once after creating an account. Since it is against the TOS to create multiple accounts, there is no point in Google sending the Promotional Code. Also I don't have any clue why the Promotional Code option is available in my Adwords Billing page when Google knows that I have already used a Promotional Code. Why not such a big company as Google implement a check condition while sending out the promo code ? and save time for its customers !