Check and document import warnings

[griff-rees]

Successful runs of create_adjacent_tables.py yields warnings of duplicates. Documenting and checking these warnings would help address potential issues with correctness.

[griff-rees]

Example warnings:

...
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: 2 newspapers found with NLP 0000174 -- keeping first
Warning: Could not find NLP 0000373 in newspaper data
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: 2 newspapers found with NLP 0000153 -- keeping first
Warning: Could not find NLP 0000668 in newspaper data
Warning: Could not find NLP 0000669 in newspaper data
Warning: 2 newspapers found with NLP 0000070 -- keeping first
Warning: 2 newspapers found with NLP 0002642 -- keeping first
Warning: 2 newspapers found with NLP 0002642 -- keeping first
Warning: 2 newspapers found with NLP 0002642 -- keeping first
...