The one time for each piece. So, you will eventually give your used space downloaded at least one time for free.
This is my understanding of the process and I can be wrong.
@littleskunk can correct me
I think so, this was the case while ago: Download failed: file does not exist - #4 by Alexey
I can’t be sure, because I’m too lazy to check the code
Topic for the Storage Node feature requests - voting - Storj Community Forum (official)