How to fix Error 6904 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 6904 occurs when SQL Server’s XML parser encounters malformed XML data containing the same attribute name declared twice within a single element tag. This typically happens due to corrupted data during an INSERT/UPDATE operation, application-level string concatenation errors when building XML payloads, or encoding mismatches when importing XML from external sources like SSIS packages or linked servers. The underlying XML schema validation engine in SQL Server 2025 strictly enforces the W3C XML specification, which prohibits duplicate attributes within the same node.

Step-by-Step Resolution Guide

Step 1: Identify the Source of the Malformed XML

  • Run the failing query in SSMS and capture the full error message, which includes the duplicate attribute name in the %s placeholder.
  • Check the application layer (C#, Java, or Python connector) generating the XML string for improper concatenation logic, especially in loops that append attributes dynamically.
  • Review SSIS package data flows or ETL scripts if the XML originates from bcp or BULK INSERT operations.

Step 2: Validate and Sanitize the XML Payload

  • Extract the offending XML string using SELECT and copy it into a validator tool like XML Notepad or an online XML linter to pinpoint the exact duplicate node.
  • Use SELECT TRY_CAST(@xmlVariable AS XML) in a T-SQL script to isolate which row or batch is causing the parsing failure before the full transaction runs.
  • Modify the source application code to use parameterized XML builders (e.g., XmlWriter in .NET) instead of manual string concatenation, which prevents duplicate attribute injection entirely.

Step 3: Adjust Schema Validation and Retry the Operation

  • If using an XML Schema Collection (XSD), open SQL Server Management Studio and navigate to Databases > [YourDB] > Programmability > Types > XML Schema Collections to ensure the schema definition doesn’t inadvertently allow duplicate keys.
  • Re-run the INSERT INTO TableName (XmlColumn) VALUES (@CleanedXml) statement with the sanitized data.
  • Restart the SQL Server (MSSQLSERVER) service via services.msc on Windows Server 2025 only if the error persists due to a cached execution plan holding invalid schema metadata.

Alternative or Verification Steps

  • Verify the Fix: Execute SELECT * FROM sys.dm_exec_requests WHERE status = 'suspended' to confirm no XML parsing threads are hanging, then re-run the original query to ensure Error 6904 no longer appears in the SQL Server Error Log (Management > SQL Server Logs in SSMS).
  • Alternative Solution: If the error persists despite cleaning the XML, bypass strict validation temporarily by casting the data as NVARCHAR(MAX) first, then use REPLACE() functions in T-SQL to strip duplicate attribute patterns via regex before casting back to the XML data type.