diff --git a/core/src/main/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolver.java b/core/src/main/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolver.java
index f75bb8312e8ca..a9725550a5fe6 100644
--- a/core/src/main/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolver.java
+++ b/core/src/main/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolver.java
@@ -72,7 +72,7 @@ public String[] concreteIndices(ClusterState state, IndicesRequest request) {
     }
 
     /**
-     * Translates the provided index expression into actual concrete indices.
+     * Translates the provided index expression into actual concrete indices, properly deduplicated.
      *
      * @param state             the cluster state containing all the data to resolve to expressions to concrete indices
      * @param options           defines how the aliases or indices need to be resolved to concrete indices
@@ -90,7 +90,7 @@ public String[] concreteIndices(ClusterState state, IndicesOptions options, Stri
     }
 
     /**
-     * Translates the provided index expression into actual concrete indices.
+     * Translates the provided index expression into actual concrete indices, properly deduplicated.
      *
      * @param state             the cluster state containing all the data to resolve to expressions to concrete indices
      * @param options           defines how the aliases or indices need to be resolved to concrete indices
@@ -137,7 +137,7 @@ String[] concreteIndices(Context context, String... indexExpressions) {
             }
         }
 
-        List<String> concreteIndices = new ArrayList<>(expressions.size());
+        final Set<String> concreteIndices = new HashSet<>(expressions.size());
         for (String expression : expressions) {
             AliasOrIndex aliasOrIndex = metaData.getAliasAndIndexLookup().get(expression);
             if (aliasOrIndex == null) {
diff --git a/core/src/test/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolverTests.java b/core/src/test/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolverTests.java
index e8459b48002c8..b074811d4a78a 100644
--- a/core/src/test/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolverTests.java
+++ b/core/src/test/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolverTests.java
@@ -868,6 +868,19 @@ public void testIndexOptions_failClosedIndicesAndAliases() {
         assertThat(results, arrayContainingInAnyOrder("foo1-closed", "foo2-closed", "foo3"));
     }
 
+    public void testDedupConcreteIndices() {
+        MetaData.Builder mdBuilder = MetaData.builder()
+                .put(indexBuilder("index1").putAlias(AliasMetaData.builder("alias1")));
+        ClusterState state = ClusterState.builder(new ClusterName("_name")).metaData(mdBuilder).build();
+        IndicesOptions[] indicesOptions = new IndicesOptions[]{ IndicesOptions.strictExpandOpen(), IndicesOptions.strictExpand(),
+                IndicesOptions.lenientExpandOpen(), IndicesOptions.strictExpandOpenAndForbidClosed()};
+        for (IndicesOptions options : indicesOptions) {
+            IndexNameExpressionResolver.Context context = new IndexNameExpressionResolver.Context(state, options);
+            String[] results = indexNameExpressionResolver.concreteIndices(context, "index1", "index1", "alias1");
+            assertThat(results, equalTo(new String[]{"index1"}));
+        }
+    }
+
     private MetaData metaDataBuilder(String... indices) {
         MetaData.Builder mdBuilder = MetaData.builder();
         for (String concreteIndex : indices) {