fix(material/schematics): don't insert duplicate @use statements in themingApi (#22755)

Author: jelbourn

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

@@ -207,6 +207,12 @@ 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.
+  const alreadyImportedPattern = new RegExp(`@use +['"]${importPath}['"]`, 'g');
+  if (alreadyImportedPattern.test(content)) {
+    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

@@ -737,4 +737,21 @@ describe('v12 theming API migration', () => {
       `@include mat.core();`,
     ]);
   });
+
+  it('should not add duplicate @use statements', async () => {
+    writeLines(THEME_PATH, [
+      `@use '~@angular/material' as mat;`,
+      `@import '~@angular/material/theming';`,
+      `$something: mat.$red-palette;`,
+      `$another: $mat-pink;`,
+    ]);
+
+    await runMigration();
+
+    expect(splitFile(THEME_PATH)).toEqual([
+      `@use '~@angular/material' as mat;`,
+      `$something: mat.$red-palette;`,
+      `$another: mat.$pink-palette;`,
+    ]);
+  });
 });