This is a diagnostic tool to ease maintaining a Zotero library.
It implements some repetitive, simple, but annoyingly repetitive tasks such as:
- List all duplicate items and/or merge them.
- List all items with no pdf files
- List all items with duplicate pdf files and/or delete them (but one).
- List standanlone items
- List items with some flaws, e.g. missing doi/isbn numbers or "ill-formmed".
- Update and/or delete some tags
- ...
(click on the picture)
Before
- 11 Items
- 2 Items are duplicates
- 5 Items have duplicate pdf files
After
- 9 Items
- 0 Items are duplicates
- 0 Items have duplicate pdf files
For read-only operations, the app is quite fast.
The UI needs some polishing. The syncing and loading can be put together. Might need some advanced Streamlite skills, though 
The notebooks use Pyzotero documentation. But they are kinda absolete. You should use the Streamlite-App in the link above.
Some parts of the merging function are adapted from zotero-cleanup.