Avoid duplicated records from cross-set fetching

[david-caro]

We fetch set-by-set to allow per-set 'last harvest', so you can
harvest individual sets from the last time that set was harvested,
as opposed to the last time any set was.

• I have all the information that I need (if not, move to RFC and look for it).
• I linked the related issue(s) in the corresponding commit logs.
• I wrote good commit log messages.
• My code follows the code style of this project.
• I've added any new docs if API/utils methods were added.
• I have updated the existing documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.