Bug: MDC / fluent key-value entries named message, level, timestamp or error produce duplicate JSON members (LambdaJsonEncoder) #2724

--- a/packages/logger/src/constants.ts
+++ b/packages/logger/src/constants.ts
@@ -37,4 +37,17 @@ const LogLevelThreshold = {
   SILENT: 28,
 } as const;
 
-export { LogJsonIndent, LogLevel, LogLevelThreshold };
+/**
+ * Reserved keys that are included in every log item when using the default log formatter.
+ *
+ * These keys are reserved and cannot be overwritten by custom log attributes.
+ */
+const ReservedKeys = [
+  'level',
+  'message',
+  'sampling_rate',
+  'service',
+  'timestamp',
+];
+
+export { LogJsonIndent, LogLevel, LogLevelThreshold, ReservedKeys };

--- a/packages/logger/src/Logger.ts
+++ b/packages/logger/src/Logger.ts
@@ -688,70 +700,118 @@ class Logger extends Utility implements LoggerInterface {
...
+  #processExtraObject(
+    item: Record<string, unknown>,
+    additionalAttributes: LogAttributes
+  ): void {
+    for (const [key, value] of Object.entries(item)) {
+      if (!this.#checkReservedKeyAndWarn(key)) {
+        additionalAttributes[key] = value;
+      }
+    }
+  }
+
+  /**
+   * Check if a given key is reserved and warn the user if it is.
+   *
+   * @param key - The key to check
+   */
+  #checkReservedKeyAndWarn(key: string): boolean {
+    if (ReservedKeys.includes(key)) {
+      this.warn(`The key "${key}" is a reserved key and will be dropped.`);
+      return true;
+    }
+    return false;
   }

diff --git a/docs/core/logging.md b/docs/core/logging.md
--- a/docs/core/logging.md
+++ b/docs/core/logging.md
@@ -744,7 +744,7 @@ Mapped Diagnostic Context (MDC) is essentially a Key-Value store. It is supporte
     Always set additional keys as part of your handler method to ensure they have the latest value, or explicitly clear them with [`clearState=true`](#clearing-state).
 
 ???+ warning "Do not add reserved keys to MDC"
-    Avoid adding any of the keys listed in [standard structured keys](#standard-structured-keys) and [additional structured keys](#additional-structured-keys) to your MDC. This may cause unindented behavior and will overwrite the context set by the Logger. Unlike with `StructuredArguments`, the Logger will **not** ignore reserved keys set via MDC.
+    Avoid adding any of the keys listed in [standard structured keys](#standard-structured-keys) and [additional structured keys](#additional-structured-keys) to your MDC. This may cause unindented behavior and will overwrite the context set by the Logger. Unlike with `StructuredArguments`, the Logger will **not** ignore reserved keys set via MDC, except `level`, `message`, `timestamp` and `error`, which the Logback `LambdaJsonEncoder` ignores in MDC and SLF4J fluent key-value entries.
 
 
 ### Removing additional keys
diff --git a/powertools-logging/powertools-logging-logback/src/main/java/software/amazon/lambda/powertools/logging/logback/LambdaJsonEncoder.java b/powertools-logging/powertools-logging-logback/src/main/java/software/amazon/lambda/powertools/logging/logback/LambdaJsonEncoder.java
--- a/powertools-logging/powertools-logging-logback/src/main/java/software/amazon/lambda/powertools/logging/logback/LambdaJsonEncoder.java
+++ b/powertools-logging/powertools-logging-logback/src/main/java/software/amazon/lambda/powertools/logging/logback/LambdaJsonEncoder.java
@@ -44,6 +44,8 @@ public class LambdaJsonEncoder extends EncoderBase<ILoggingEvent> {
     protected static final String EXCEPTION_CLASS_ATTR_NAME = "name";
     protected static final String EXCEPTION_STACK_ATTR_NAME = "stack";
     protected static final String EXCEPTION_ATTR_NAME = "error";
+    private static final Set<String> RESERVED_ATTR_NAMES = Set.of(LEVEL_ATTR_NAME, FORMATTED_MESSAGE_ATTR_NAME,
+            TIMESTAMP_ATTR_NAME, EXCEPTION_ATTR_NAME);
 
     private final ThrowableProxyConverter throwableProxyConverter = new ThrowableProxyConverter();
     protected ThrowableHandlingConverter throwableConverter = null;
@@ -80,6 +82,8 @@ public class LambdaJsonEncoder extends EncoderBase<ILoggingEvent> {
 
             TreeMap<String, String> sortedMap = new TreeMap<>(event.getMDCPropertyMap());
             sortedMap.putAll(getKeyValuePairs(event));
+            // the encoder writes these fields itself, MDC or key-value entries would duplicate them
+            sortedMap.keySet().removeAll(RESERVED_ATTR_NAMES);
             serializePowertools(sortedMap, serializer);
 
             serializeMDCEntries(sortedMap, serializer);