fix(compiler): throw error for duplicate template references

Adds an error if a reference is used more than once on the same element (e.g. `<div #a #a>`).
We used to have this error in ViewEngine, but it wasn't ported over to Ivy.

Fixes #40536.

packages/compiler/src/render3/r3_template_transform.ts

@@ -458,6 +458,8 @@ class HtmlAstToIvyAst implements html.Visitor {
       this.reportError(`"-" is not allowed in reference names`, sourceSpan);
     } else if (identifier.length === 0) {
       this.reportError(`Reference does not have a name`, sourceSpan);
+    } else if (references.some(reference => reference.name === identifier)) {
+      this.reportError(`Reference "#${identifier}" is defined several times`, sourceSpan);
     }
 
     references.push(new t.Reference(identifier, value, sourceSpan, keySpan, valueSpan));

packages/compiler/test/render3/r3_template_transform_spec.ts

@@ -273,6 +273,11 @@ describe('R3 template transform', () => {
       ]);
     });
 
+    it('should report an error if a reference is used multiple times on the same template', () => {
+      expect(() => parse('<ng-template #a #a></ng-template>'))
+          .toThrowError(/Reference "#a" is defined several times/);
+    });
+
     it('should parse variables via let-...', () => {
       expectFromHtml('<ng-template let-a="b"></ng-template>').toEqual([
         ['Template'],
@@ -463,6 +468,11 @@ describe('R3 template transform', () => {
     it('should report missing reference names', () => {
       expect(() => parse('<div #></div>')).toThrowError(/Reference does not have a name/);
     });
+
+    it('should report an error if a reference is used multiple times on the same element', () => {
+      expect(() => parse('<div #a #a></div>'))
+          .toThrowError(/Reference "#a" is defined several times/);
+    });
   });
 
   describe('literal attribute', () => {