Who ever told you that was misinformed. I've had the promo since the S3, and recently my dropbox had expired and dropped down to the usual 5gb when I did all the bonus stuff to gain the extra space. I got the S4, Note 2, as well as the Note 3 during the time, and was told that the promo cannot be stacked. When I logged into my dropbox account when I got the S5 2 weeks ago, I got a notification that my dropbox had been upgraded again to 52gb now. So you will have to wait until it expires, then you can use the same account to gain the extra space.
Now I have 56.5GB on my account!
But when I logged in with a new account (even in my months old Note 3, which had been heavily used in my old dropbox account) it did award me the bonus.
Well I guess I will have to move accounts (and I had 12GB from other promos and stuff). Can't complain. And I guess in two years time, they will be giving much more than that for free.