diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetaData.java b/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetaData.java
index f1dd843d798..5d23971dddb 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetaData.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetaData.java
@@ -1531,14 +1531,14 @@ public class IndexMetaData implements Diffable<IndexMetaData>, ToXContentFragmen
         if (sourceNumberOfShards < targetNumberOfShards) { // split
             factor = targetNumberOfShards / sourceNumberOfShards;
             if (factor * sourceNumberOfShards != targetNumberOfShards || factor <= 1) {
-                throw new IllegalArgumentException("the number of source shards [" + sourceNumberOfShards + "] must be a must be a " +
+                throw new IllegalArgumentException("the number of source shards [" + sourceNumberOfShards + "] must be a " +
                     "factor of ["
                     + targetNumberOfShards + "]");
             }
         } else if (sourceNumberOfShards > targetNumberOfShards) { // shrink
             factor = sourceNumberOfShards / targetNumberOfShards;
             if (factor * targetNumberOfShards != sourceNumberOfShards || factor <= 1) {
-                throw new IllegalArgumentException("the number of source shards [" + sourceNumberOfShards + "] must be a must be a " +
+                throw new IllegalArgumentException("the number of source shards [" + sourceNumberOfShards + "] must be a " +
                     "multiple of ["
                     + targetNumberOfShards + "]");
             }
diff --git a/server/src/test/java/org/elasticsearch/cluster/metadata/IndexMetaDataTests.java b/server/src/test/java/org/elasticsearch/cluster/metadata/IndexMetaDataTests.java
index 393f7f6b1d4..1fdea596afb 100644
--- a/server/src/test/java/org/elasticsearch/cluster/metadata/IndexMetaDataTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/metadata/IndexMetaDataTests.java
@@ -227,7 +227,7 @@ public class IndexMetaDataTests extends ESTestCase {
         assertEquals("the number of target shards (0) must be greater than the shard id: 0",
             expectThrows(IllegalArgumentException.class, () -> IndexMetaData.selectSplitShard(0, metaData, 0)).getMessage());
 
-        assertEquals("the number of source shards [2] must be a must be a factor of [3]",
+        assertEquals("the number of source shards [2] must be a factor of [3]",
             expectThrows(IllegalArgumentException.class, () -> IndexMetaData.selectSplitShard(0, metaData, 3)).getMessage());
 
         assertEquals("the number of routing shards [4] must be a multiple of the target shards [8]",
@@ -285,6 +285,6 @@ public class IndexMetaDataTests extends ESTestCase {
         Settings notAFactorySettings = Settings.builder().put("index.number_of_shards", 2).put("index.number_of_routing_shards", 3).build();
         iae = expectThrows(IllegalArgumentException.class,
             () -> IndexMetaData.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.get(notAFactorySettings));
-        assertEquals("the number of source shards [2] must be a must be a factor of [3]", iae.getMessage());
+        assertEquals("the number of source shards [2] must be a factor of [3]", iae.getMessage());
     }
 }
diff --git a/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexServiceTests.java b/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexServiceTests.java
index 6cbd83e5b24..ec89e085f07 100644
--- a/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexServiceTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexServiceTests.java
@@ -154,7 +154,7 @@ public class MetaDataCreateIndexServiceTests extends ESTestCase {
                 MetaDataCreateIndexService.validateShrinkIndex(state, "source", Collections.emptySet(), "target", targetSettings)
 
             ).getMessage());
-        assertEquals("the number of source shards [8] must be a must be a multiple of [3]",
+        assertEquals("the number of source shards [8] must be a multiple of [3]",
             expectThrows(IllegalArgumentException.class, () ->
                     MetaDataCreateIndexService.validateShrinkIndex(createClusterState("source", 8, randomIntBetween(0, 10),
                         Settings.builder().put("index.blocks.write", true).build()), "source", Collections.emptySet(), "target",
@@ -221,7 +221,7 @@ public class MetaDataCreateIndexServiceTests extends ESTestCase {
             ).getMessage());
 
 
-        assertEquals("the number of source shards [3] must be a must be a factor of [4]",
+        assertEquals("the number of source shards [3] must be a factor of [4]",
             expectThrows(IllegalArgumentException.class, () ->
                 MetaDataCreateIndexService.validateSplitIndex(createClusterState("source", 3, randomIntBetween(0, 10),
                     Settings.builder().put("index.blocks.write", true).build()), "source", Collections.emptySet(), "target",