Add breaking change documentation for Console JSON logging duplicate messages

docs/core/compatibility/10.0.md

@@ -41,6 +41,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change      | Introduced version |
 |-------|---------------------|--------------------|
+| [Message no longer duplicated in Console log output](extensions/10.0/console-json-logging-duplicate-messages.md) | Behavioral change | Preview 7 |
 | [ProviderAliasAttribute moved to Microsoft.Extensions.Logging.Abstractions assembly](extensions/10.0/provideraliasattribute-moved-assembly.md) | Source incompatible | Preview 4 |
 | [Removed DynamicallyAccessedMembers annotation from trim-unsafe Microsoft.Extensions.Configuration code](extensions/10.0/dynamically-accessed-members-configuration.md) | Binary incompatible | Preview 6 |
 

docs/core/compatibility/extensions/10.0/console-json-logging-duplicate-messages.md

@@ -0,0 +1,79 @@
+---
+title: "Breaking change: Message no longer duplicated in Console log output"
+description: "Learn about the breaking change in .NET 10 where `Message` is no longer duplicated in Console log output using the JSON formatter."
+ms.date: 08/07/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/47006
+---
+
+# Message no longer duplicated in Console log output
+
+When logging to the console using the JSON formatter, log messages are no longer duplicated in the log output. Previously, messages typically appeared three times: once as the top-level `Message`, again within the `State` object, and a third time as the original format string.
+
+## Version introduced
+
+.NET 10 Preview 7
+
+## Previous behavior
+
+Previously, when using a console logger configured with the JSON formatter, log messages were duplicated in the output. For example, the code `logger.LogInformation("This is an information message.");` produced the following output:
+
+```json
+{
+  "EventId": 0,
+  "LogLevel": "Information",
+  "Category": "Program",
+  "Message": "This is an information message.",
+  "State": {
+    "Message": "This is an information message.",
+    "{OriginalFormat}": "This is an information message."
+  }
+}
+```
+
+As you can see, `Message` appears twice: once as the top-level `Message` and again inside the `State` object.
+
+## New behavior
+
+Starting in .NET 10, `Message` appears only at the top level and not inside the `State` object (typically). The log output looks like this:
+
+```json
+{
+  "EventId": 0,
+  "LogLevel": "Information",
+  "Category": "Program",
+  "Message": "This is an information message.",
+  "State": {
+    "{OriginalFormat}": "This is an information message."
+  }
+}
+```
+
+## Type of breaking change
+
+This is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+The goal of this change is to reduce unnecessary logging overhead by eliminating duplicate content. By avoiding repeated formatting of the same message, the change helps:
+
+- Minimize log output size.
+- Reduce confusion caused by redundant information.
+- Improve performance by preventing multiple formatting operations for the same message.
+
+Overall, this results in cleaner, more efficient, and easier-to-read logs.
+
+## Recommended action
+
+If you previously parsed the logging output to extract the `Message` from within the `State` object, it's safe to use the top-level `Message` instead, now that duplication has been removed.
+
+> [!NOTE]
+> In some cases, a `Message` might still appear within the `State` object. This typically happens when its content differs from the top-level `Message`.
+
+## Affected APIs
+
+- <xref:Microsoft.Extensions.Logging.ConsoleLoggerExtensions.AddConsole%2A?displayProperty=fullName>
+- <xref:Microsoft.Extensions.Logging.ConsoleLoggerExtensions.AddConsoleFormatter*?displayProperty=fullName>
+- <xref:Microsoft.Extensions.Logging.ConsoleLoggerExtensions.AddJsonConsole*?displayProperty=fullName>
+- <xref:Microsoft.Extensions.Logging.ConsoleLoggerExtensions.AddSimpleConsole*?displayProperty=fullName>
+- <xref:Microsoft.Extensions.Logging.ConsoleLoggerExtensions.AddSystemdConsole*?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -50,9 +50,11 @@ items:
                 href: cryptography/10.0/x509-publickey-null.md
           - name: Extensions
             items:
-              - name: "ProviderAliasAttribute moved to Microsoft.Extensions.Logging.Abstractions assembly"
+              - name: Message no longer duplicated in Console log output
+                href: extensions/10.0/console-json-logging-duplicate-messages.md
+              - name: ProviderAliasAttribute moved to Microsoft.Extensions.Logging.Abstractions assembly
                 href: extensions/10.0/provideraliasattribute-moved-assembly.md
-              - name: "Removed DynamicallyAccessedMembers annotation from trim-unsafe Microsoft.Extensions.Configuration code"
+              - name: Removed DynamicallyAccessedMembers annotation from trim-unsafe Microsoft.Extensions.Configuration code
                 href: extensions/10.0/dynamically-accessed-members-configuration.md
           - name: Globalization
             items: