fix(ec2): prevent deduplication of init command args

packages/aws-cdk-lib/aws-ec2/lib/cfn-init.ts

@@ -311,10 +311,14 @@ function deepMerge(target?: Record<string, any>, src?: Record<string, any>) {
       if (target[key] && !Array.isArray(target[key])) {
         throw new Error(`Trying to merge array [${value}] into a non-array '${target[key]}'`);
       }
-      target[key] = Array.from(new Set([
-        ...target[key] ?? [],
-        ...value,
-      ]));
+      if (key != 'command') {  // don't deduplicate command arguments
+        target[key] = Array.from(new Set([
+          ...target[key] ?? [],
+          ...value,
+        ]));
+      } else {
+        target[key] = value;
+      }
       continue;
     }
     if (typeof value === 'object' && value) {

packages/aws-cdk-lib/aws-ec2/test/cfn-init.test.ts

@@ -136,6 +136,44 @@ test('empty configs are not rendered', () => {
   });
 });
 
+test('duplicate config arguments not deduplicated', () => {
+  //GIVEN
+  const config = new ec2.InitConfig([
+    ec2.InitCommand.argvCommand([
+      'useradd', '-u', '1001', '-g', '1001', 'eguser',
+    ]),
+    ec2.InitCommand.argvCommand([
+      'useradd', '-a', '-u', '1001', '-g', '1001', 'eguser',
+    ]),
+  ]);
+
+  // WHEN
+  const init = ec2.CloudFormationInit.fromConfigSets({
+    configSets: {default: ['config']},
+    configs: {config},
+  });
+  init.attach(resource, linuxOptions());
+
+  // THEN
+  expectMetadataLike({
+    'AWS::CloudFormation::Init': {
+      configSets: {
+        default: ['config'],
+      },
+      config: {
+        commands: {
+          '000': {
+            command: ['useradd', '-u', '1001', '-g', '1001', 'eguser'],
+          },
+          '001': {
+            command: ['useradd', '-a', '-u', '1001', '-g', '1001', 'eguser'],
+          },
+        },
+      },
+    },
+  });
+});
+
 describe('userdata', () => {
   let simpleInit: ec2.CloudFormationInit;
   beforeEach(() => {