UNIQUE constraint failed: books_instances.book_id, books_instances.instance_id #8

[2019-01-30 20:34:17,596] ERROR in app: Exception on /uploads/21_Spektrum_Community_Report_Oct2018_CCBY30DE.pdf [GET]
Traceback (most recent call last):
  File "/home/xppl/.local/share/virtualenvs/xppl-UCsE39NU/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/home/xppl/.local/share/virtualenvs/xppl-UCsE39NU/lib/python3.5/site-packages/sqlalchemy/engine/default.py", line 509, in do_execute
    cursor.execute(statement, parameters)
  File "/home/xppl/.local/share/virtualenvs/xppl-UCsE39NU/lib/python3.5/site-packages/pyrqlite/cursors.py", line 178, in execute
    raise Error(json.dumps(item))
sqlite3.Error: {"error": "UNIQUE constraint failed: books_instances.book_id, books_instances.instance_id"}

Also saw:

sqlalchemy.exc.DBAPIError: (sqlite3.Error) {"error": "UNIQUE constraint failed: books_instances.book_id, books_instances.instance_id"} [SQL: 'INSERT INTO books_instances (book_id, instance_id) VALUES (?, ?)'] [parameters: (22, 296)] (Background on this error at: http://sqlalche.me/e/dbapi)
[2019-01-30 20:35:56,093] ERROR in app: Exception on /updates [GET]