diff --git a/server/src/main/java/org/elasticsearch/repositories/ShardGenerations.java b/server/src/main/java/org/elasticsearch/repositories/ShardGenerations.java
index 6e8ae1956f945..e42552d3e5f3c 100644
--- a/server/src/main/java/org/elasticsearch/repositories/ShardGenerations.java
+++ b/server/src/main/java/org/elasticsearch/repositories/ShardGenerations.java
@@ -9,6 +9,7 @@
 package org.elasticsearch.repositories;
 
 import org.elasticsearch.cluster.SnapshotsInProgress;
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.index.snapshots.IndexShardSnapshotStatus;
 
@@ -219,12 +220,22 @@ public Builder put(IndexId indexId, int shardId, SnapshotsInProgress.ShardSnapsh
         }
 
         public Builder put(IndexId indexId, int shardId, ShardGeneration generation) {
+            assert noDuplicateIndicesWithSameName(indexId);
             ShardGeneration existingGeneration = generations.computeIfAbsent(indexId, i -> new HashMap<>()).put(shardId, generation);
             assert generation != null || existingGeneration == null
                 : "must not overwrite existing generation with null generation [" + existingGeneration + "]";
             return this;
         }
 
+        private boolean noDuplicateIndicesWithSameName(IndexId newId) {
+            for (IndexId id : generations.keySet()) {
+                if (id.getName().equals(newId.getName()) && id.equals(newId) == false) {
+                    assert false : Strings.format("Unable to add: %s. There's another index id with the same name: %s", newId, id);
+                }
+            }
+            return true;
+        }
+
         public ShardGenerations build() {
             return new ShardGenerations(generations.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, entry -> {
                 final Set<Integer> shardIds = entry.getValue().keySet();