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
} else if (token.isValue()) {
                    if ("version".equals(currentFieldName)) {
                        builder.version = parser.longValue();
                    } else if ("cluster_uuid".equals(currentFieldName) || "uuid".equals(currentFieldName)) {
                        builder.clusterUUID = parser.text();
                    } else if ("cluster_uuid_committed".equals(currentFieldName)) {
                        builder.clusterUUIDCommitted = parser.booleanValue();
                    } else {
                        throw new IllegalArgumentException("Unexpected field [" + currentFieldName + "]");
                    }
                } else {
                    throw new IllegalArgumentException("Unexpected token " + token);
                }
            }
            XContentParserUtils.ensureExpectedToken(XContentParser.Token.END_OBJECT, parser.nextToken(), parser);
            return builder.build();
        }
        /**
         * Dedupes {@link MappingMetadata} instance from the provided indexMetadata parameter using the sha256
         * hash from the compressed source of the mapping. If there is a mapping with the same sha256 hash then
         * a new {@link IndexMetadata} is returned with the found {@link MappingMetadata} instance, otherwise
         * the {@link MappingMetadata} instance of the indexMetadata parameter is recorded and the indexMetadata
         * parameter is then returned.
         */
        private IndexMetadata dedupeMapping(IndexMetadata indexMetadata) {
            if (indexMetadata.mapping() == null) {
                return indexMetadata;
            }
            String digest = indexMetadata.mapping().getSha256();
            MappingMetadata entry = mappingsByHash.get(digest);