A transport put follow-action can be triggered for an index that is part of a data stream and already exists.
Normally this is not an issue because the restore operation will just fail because the index already exists.
If however, the index is currently closed (for example due to ILM doing a force-merge with codec change step on the index),
this causes the currently closed index to be restored into. As a result we run into a bug in the following code in

`org.elasticsearch.xpack.ccr.action.TransportPutFollowAction#createFollowerIndex`:

```java
BiConsumer<ClusterState, Metadata.Builder> updater = (currentState, mdBuilder) -> {
   DataStream localDataStream = currentState.getMetadata().dataStreams().get(remoteDataStream.getName());
   Index followerIndex = mdBuilder.get(followerIndexName).getIndex();
   assert followerIndex != null;

   DataStream updatedDataStream = updateLocalDataStream(followerIndex, localDataStream, remoteDataStream);
      mdBuilder.put(updatedDataStream);
   };
   restoreService.restoreSnapshot(restoreRequest, delegatelistener, updater);
 }
```

This code updates the local data stream when following an index without checking if that index already is part of the data stream, thus now creating two entries for the same `Index` in the data stream's indices list. Code like `org.elasticsearch.cluster.metadata.DataStream#removeBackingIndex` however assumes no duplicates and will when deleting an index, only remove it from the data stream once. Thus leaving a dangling `Index` instance that does not point to any existing index any more, corrupting the cluster state.


cc @martijnvg thanks for the help with reasoning this out
server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java
private static final Logger logger = LogManager.getLogger(Metadata.class);
    public static final Runnable ON_NEXT_INDEX_FIND_MAPPINGS_NOOP = () -> {};
    public static final String ALL = "_all";
    public static final String UNKNOWN_CLUSTER_UUID = "_na_";
    public enum XContentContext {
        
        API,
        
        GATEWAY,
        
        SNAPSHOT
    }
    /**
     * Indicates that this custom metadata will be returned as part of an API call but will not be persisted
     */
    public static EnumSet<XContentContext> API_ONLY = EnumSet.of(XContentContext.API);
    /**
     * Indicates that this custom metadata will be returned as part of an API call and will be persisted between
     * node restarts, but will not be a part of a snapshot global state
     */
    public static EnumSet<XContentContext> API_AND_GATEWAY = EnumSet.of(XContentContext.API, XContentContext.GATEWAY);
    /**
     * Indicates that this custom metadata will be returned as part of an API call and stored as a part of
     * a snapshot global state, but will not be persisted between node restarts
     */
    public static EnumSet<XContentContext> API_AND_SNAPSHOT = EnumSet.of(XContentContext.API, XContentContext.SNAPSHOT);