Unfortunately, GoSurf (30 days) does not stack with another GoSurf (30 days). As far as I remember there is a 10 GB GoSurf (GoSurf 1799), but I am not sure if it is a part of the My Super Plan Combos. In addition, the Spotify code provided won't stack as well. So if you avail of GoSurf, it's just for 6 months. But there is a Spotify promo for PHP 299 a month. That would give you Premium Spotify + A Dedicated Data Bucket just for Spotify, so you don't need to worry about Spotify using your data allocation.
Hope this helps.
You may want to refer to GoSURF | HELP AND SUPPORT for your other queries regarding GoSurf...
For Postpaid customers:
Yes, you can register to GoSURF again. If you are subscribed to a GoSURF 30-day plan, you can only register to GoSURF non-30 day plans. This means that you can register to GoSURF 15, GoSURF 30 and GoSURF 50 even while your GoSURF 30D offer is still active.
But why would you want to subscribe (assuming it's possible) to another GoSurf999 (and spend Php999) when all you have to do is just exceed your 5Gb until you reach the anti-bill shock threshold of Php1,500 which would only be an additional Php 501.00 from your previous Php 999.00 subscription? It's like having unlimited internet without the throttling.