Duplicate import identifier when re-exporting with export star when preserveModules is true

[kamranayub]

• Rollup Version: 1.31.0-2.0.3
• Operating System (or Browser): Mac OS Mojave
• Node Version: v12.14.1

Search terms: export star, reexport, re-export, nested modules, preserveModules

When using preserveModules, we're seeing an issue handling nested module re-exporting using export * as shown below.

How Do We Reproduce?

Minimal reproduction: https://github.com/kamranayub/rollup-bug-reexporting-preserveModules

1. You can view the output in dist/index.js
2. You can also run yarn and yarn build to rebuild the output

Expected Behavior

We'd expect Rollup to de-dupe the import alias similar to how it behaves with preserveModules: false (e.g. index$1)

import * as index from './module-a/v1/index.js';
export { index as ModuleA_V1 };
import * as index$1 from './module-b/v1/index.js';
export { index$1 as ModuleB_V1 };

Actual Behavior

Rollup generates a duplicate import alias:

import * as index from './module-a/v1/index.js';
export { index as ModuleA_V1 };
import * as index from './module-b/v1/index.js';
export { index as ModuleB_V1 };

This will result in an error when consumed:

Identifier 'index' has already been declared

See: https://github.com/kamranayub/rollup-bug-reexporting-preserveModules/blob/master/dist/index.js