I added a field to a nodetype, and found out I had selected the wrong fieldtype. I removed it and added the field (with the same name) but with the correct type. I tried to rebuild, but then I suddenly had two fields with the same name and therefore got an error preventing the rebuild. It seems it doesn't actually remove the field before rebuild. The same happened again when I wanted to figure out if it might work a third time, but I ended up with three with the same name. No biggie, just a bit annoying.. :) I am using build 1521.