Revision Manager Duplicate Entry

Problem/Motivation

The revision_manager module has a race condition in QueueGenerator::enqueueIfUnique()
method. Multiple processes (cron runs, batch operations, manual triggers) can
simultaneously attempt to queue the same entity for revision cleanup, causing:

• Process A checks if key exists → FALSE
• Process B checks if key exists → FALSE
• Both processes attempt to INSERT the same primary key → Database constraint violation

Proposed resolution

Add exception handling to gracefully handle the duplicate key scenario:

• Catch Duplicate entry database exceptions
• Return FALSE to indicate item was already queued by another process
• Maintain existing error handling for other exception types