When performing a search you receive the following error in the "System Log":
Query Parser Error:[Argument_AddingDuplicate] A Search error has occurred.
A search error has occurred. any result may not be valid.
The following error may appear in "Searchlog.txt":
SearchJob failed due to unhandled exception.
System.ArgumentException: An item with the same key has already been added.
When attempting to save an "advanced search" an error appears:
Rename or delete the custom fields created. As a best practice workflow, if you prepend all your custom fields with a 'aa' or 'zz', they will be grouped into an area when you get in review. After renaming you will have to clear your browser cache.
The attached T-SQL script will check all of the case databases for custom field display names that duplicate known system field names. Any custom fields that cause this issue will have 'Custom' prepended onto the field name (e.g., a custom field named "Issues" would be renamed "CustomIssues").
The script can be run in Microsoft SQL Management Studio.
This is caused when a case contains created custom fields that have the same name as system fields.