Log bad or duplicate identifiers to a file

example from r2r: @id: doi:null

This is intentional on their part. These are not 'published' need to confirm.

Occurs on a large number of files.

sitemap_count": 53386,
  "summoned_count": 44401,
  "missing_sitemap_summon_count": 9009,

so need a :

list of just bad identifiers. We might do something with these to fix them on a per source basis
use hashmap to track duplicate identifiers in the source

gleanerio / gleaner