Detect duplicate serializer identifiers, #29100

* Detect duplicate serializer identifiers, #29100
* and expand the documentation for the serializer identifier
* and mention BaseSerializer to read the identifier from config
* same class same id is ok

akka-docs/src/main/paradox/serialization.md

@@ -82,6 +82,7 @@ Scala
 Java
 :  @@snip [SerializationDocTest.java](/akka-docs/src/test/java/jdocs/serialization/SerializationDocTest.java) { #programmatic }
 
+
 The manifest is a type hint so that the same serializer can be used for different classes.
 
 Note that when deserializing from bytes the manifest and the identifier of the serializer are needed.
@@ -119,6 +120,21 @@ Scala
 Java
 :  @@snip [SerializationDocTest.java](/akka-docs/src/test/java/jdocs/serialization/SerializationDocTest.java) { #my-own-serializer }
 
+The `identifier` must be unique. The identifier is used when selecting which serializer to use for deserialization.
+If you have accidentally configured several serializers with the same identifier that will be detected and prevent
+the `ActorSystem` from being started. It can be a hardcoded value because it must remain the same value to support
+rolling updates. 
+
+@@@ div { .group-scala }
+
+If you prefer to define the identifier in cofiguration that is supported by the `BaseSerializer` trait, which
+implements the `def identifier` by reading it from configuration based on the serializer's class name:
+
+Scala
+:  @@snip [SerializationDocSpec.scala](/akka-docs/src/test/scala/docs/serialization/SerializationDocSpec.scala) { #serialization-identifiers-config }
+
+@@@
+
 The manifest is a type hint so that the same serializer can be used for different
 classes. The manifest parameter in @scala[`fromBinary`]@java[`fromBinaryJava`] is the class of the object that
 was serialized. In `fromBinary` you can match on the class and deserialize the

akka-docs/src/test/scala/docs/serialization/SerializationDocSpec.scala

@@ -109,6 +109,21 @@ package docs.serialization {
    */
   trait JsonSerializable
 
+  object SerializerIdConfig {
+    val config =
+      """
+        #//#serialization-identifiers-config
+        akka {
+          actor {
+            serialization-identifiers {
+              "docs.serialization.MyOwnSerializer" = 1234567
+            }
+          }
+        }
+        #//#serialization-identifiers-config
+        """
+  }
+
   class SerializationDocSpec extends AkkaSpec {
     "demonstrate configuration of serialize messages" in {
       val config = ConfigFactory.parseString("""