I think Ashley's got it. But I'll state it my way to see if we find a discrepancy...
For Community, it says:
35 points required
New items from Sand Thorn list allowed (each counts for 1.5 points)
Up to 8 oddball points of community art allowed
Up to 10 points of duplication allowed from the Straw and Sand list
So this means you can look back at the Sand list and do items in there that you haven't done before. They're worth 1.5 points since the Sand list doesn't have points assigned to each task.
You can also duplicate (repeat) any BBs in Sand and Straw to keep racking up points. I hadn't thought about duplication only being allowed once... I'll have to run that past Paul to get his ruling. In my mind you could duplicate a BB as many times as you want....
Since there are 11 BBs on the "thorn list" and you did two for Sand, you could do the remaining 9 and get 13.5 points for them. Then you could duplicate up to 10 points of them, getting you to 23.5. Since Straw requires 35 points you still have 11.5 points to go and they need to be from the Straw BBs. My hunch is that it's easier to get a few Straw BBs than to do all the Sand BBs but it's up to you.