diff --git a/config-model/src/main/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidator.java b/config-model/src/main/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidator.java
index 1675bacb387..f3a61a8e45d 100644
--- a/config-model/src/main/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidator.java
+++ b/config-model/src/main/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidator.java
@@ -8,8 +8,10 @@
 import com.yahoo.vespa.model.container.ContainerModel;
 
 import java.io.Reader;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Optional;
+import java.util.Set;
 
 /**
  * Validate deployment spec (deployment.xml).
@@ -27,6 +29,7 @@ public void validate(Context context) {
         Reader deploymentReader = deployment.get();
         DeploymentSpec deploymentSpec = DeploymentSpec.fromXml(deploymentReader);
         List<ContainerModel> containers = context.model().getRoot().configModelRepo().getModels(ContainerModel.class);
+        requireUniqueInstanceIds(context, deploymentSpec.instances());
         for (DeploymentInstanceSpec instance : deploymentSpec.instances()) {
             instance.endpoints().forEach(endpoint -> {
                 requireClusterId(context, containers, instance.name(),
@@ -42,4 +45,12 @@ private static void requireClusterId(Context context, List<ContainerModel> conta
                             "' specified in deployment.xml does not match any container cluster ID");
     }
 
+    private static void requireUniqueInstanceIds(Context context, List<DeploymentInstanceSpec> instances) {
+        Set<InstanceName> instanceNames = new HashSet<>();
+        for (var instance : instances) {
+            if ( ! instanceNames.add(instance.name()))
+                context.illegal("Duplicate instance name '" + instance.name() + "' specified in deployment.xml.");
+        }
+    }
+
 }
diff --git a/config-model/src/test/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidatorTest.java b/config-model/src/test/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidatorTest.java
index c9b014d9301..6f41577d55e 100644
--- a/config-model/src/test/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidatorTest.java
+++ b/config-model/src/test/java/com/yahoo/vespa/model/application/validation/DeploymentSpecValidatorTest.java
@@ -34,6 +34,25 @@ void testEndpointNonExistentContainerId() {
                 "deployment.xml does not match any container cluster ID", deploymentXml);
     }
 
+    @Test
+    void requireUniqueInstanceId() {
+        String deploymentXml = """
+                    <deployment version="1.0" cloud-account="aws:010438471985">
+                    <instance id="default" tags="search">
+                        <prod>
+                            <region>aws-us-west-2a</region>
+                        </prod>
+                    </instance>
+                    <instance id="default" tags="canary">
+                        <prod>
+                            <region>aws-us-west-2a</region>
+                        </prod>
+                    </instance>
+                </deployment>
+                """;
+        assertValidationError("Duplicate instance name 'default' specified in deployment.xml.", deploymentXml);
+    }
+
     private static void assertValidationError(String message, String deploymentXml) {
         var simpleHosts = "<?xml version=\"1.0\" encoding=\"utf-8\" ?>" +
                           "<hosts>  " +