fix(material/schematics): don't insert duplicate @use statements in themingApi

In the case where a file has mixed legacy and new theming API usage, the script would
add an extra duplicate `@use` statement.

Author: jelbourn

src/material/schematics/ng-update/migrations/theming-api-v12/migration.ts

@@ -205,6 +205,11 @@ function renameSymbols(content: string,
 /** Inserts an `@use` statement in a string. */
 function insertUseStatement(content: string, importPath: string, importsToIgnore: string[],
                             namespace: string): string {
+  // If the content already has the `@use` import, we don't need to add anything.
+  if (content.includes(`@use '${importPath}'`)) {
+    return content;
+  }
+
   // We want to find the first import that isn't in the list of ignored imports or find nothing,
   // because the imports being replaced might be the only ones in the file and they can be further
   // down. An easy way to do this is to replace the imports with a random character and run

src/material/schematics/ng-update/test-cases/v12/misc/theming-api-v12.spec.ts

@@ -734,4 +734,26 @@ describe('v12 theming API migration', () => {
       `@include mat.core();`,
     ]);
   });
+
+  it('should not add duplicate @use statements', async () => {
+    const originalContent = [
+      `@use '~@angular/material' as mat;`,
+      `@import '~@angular/material/theming';`,
+      `$something: mat.$red-palette;`,
+      `$another: $mat-pink;`,
+    ].join('\n');
+
+    const migratedContent = migrateFileContent(
+        originalContent,
+        '~@angular/material/',
+        '~@angular/cdk/',
+        '~@angular/material',
+        '~@angular/cdk');
+
+    expect(migratedContent).toBe([
+      `@use '~@angular/material' as mat;`,
+      `$something: mat.$red-palette;`,
+      `$another: mat.$pink-palette;`,
+    ].join('\n'));
+  });
 });