2x duplication with 3 drives? Maximum capacity?

I have 3 x 5TB drives added to my pool, and my max capacity says 13.6TB. I have set 2x duplication for the entire pool. Questions:

1. How does 2x duplication work with 3 drives? Am I protected if one of the three drives fails?
2. I have 13.6TB in the pool, but I was able to put in 12.9TB worth of files into the pool somehow. It says I have roughly 12.9TB duplicated files and 600GB left. If it's 2x duplication (~25TB), should it have prompted me for more space or something when I reach half of that? Really confused.