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When a Microsoft Exchange Server database file becomes corrupted, it can trigger a range of JET engine errors, each unique to the underlying cause of the corruption. Fortunately, many of these JET engine errors can be effectively addressed by utilizing eseutil.exe, an essential built-in utility within Exchange Server designed specifically for resolving common issues related to MS Exchange Server databases. This utility meticulously examines the database file, actively identifying and rectifying errors where possible. However, it’s worth noting that there are certain JET engine errors that elude resolution through the eseutil.exe utility. One such elusive error is:

-1605 0xFFFFF9BB JET_errKeyDuplicate Illegal duplicate key 4294965691

When the error message mentioned earlier surfaces, it disrupts your ability to proceed with MS Exchange Server operations. This particular error typically occurs when the Eseutil tool inaccurately reconstructs B-trees, leading to a situation where the Exchange database cannot be remounted.

Actual reason behind the error

The issue at hand cannot be resolved using the Eseutil.exe tool. In fact, persistently attempting to use Eseutil.exe after encountering this error message can potentially lead to data loss and wasted time, exacerbating the problem rather than solving it. This occurs because the /p switch within the Eseutil utility attempts to repair all system tables and indexes. When encountering severely corrupted tables or indexes, the repair process discards them. As a consequence, ESE Event IDs are generated, indicating the loss of data pages during the repair operation. This data loss is irrevocable and is likely to exacerbate existing issues.


Exchange Server experts highly advise the creation of comprehensive backups for Exchange Server database files. This proactive measure ensures that in the event of JET errors, data recovery becomes a straightforward process. If you possess a complete backup of the afflicted Exchange Server database file, restoring it allows you to resume working with Exchange Server seamlessly, free from any data loss concerns. However, should you lack a backup, the assistance of third-party Exchange Database Recovery software becomes essential.

EDB Recovery software performs a thorough scan of corrupted Exchange Server database files, even in the presence of Jet Engine errors, and proficiently restores them to their original state. Once the scanning process is completed, the software allows users to save the recovered data to a location of their choice. Kernel for Exchange Server recovery is a powerful third-party software solution meticulously crafted to offer comprehensive Exchange Server database recovery services. It is compatible with nearly all versions of MS Exchange Server, spanning from Exchange Server 5.0 to 2019, ensuring a wide range of support for various environments and scenarios.


Certain Exchange errors, such as “JET_errKeyDuplicate Illegal duplicate key,” are beyond the scope of repair using the eseutil utility due to the inherent risk of data loss. In such critical situations, the optimal solution is to employ a robust recovery tool like Kernel for Exchange Server.

Kernel for Exchange Server