Duplicate code when running SWC with --output-file and --watch on WIndows #10524

Describe the bug

When running SWC like this

npx swc src/app/v1/simple.js --out-file public/v1/output.js --watch

The first compile is a proper 1:1 copy of the .js file. However whenever I change anything in the source file and save it, the target file now contains the changes I made, but prepended with the original source file.

Note that working with --output-dir instead of --output-file does not have this problem.

Also normal compiles without --watch work fine.

Input code

npx swc src/app/v1/simple.js --out-file public/v1/simple.js --watch

Config

{}

Link to the code that reproduces this issue

https://play.swc.rs/?version=1.11.29&code=H4sIAAAAAAAAA0vOzyvOz0nVy8lP11DySM3JyddRKM8vyklRVFTStAYAhtlSch4AAAA%3D&config=H4sIAAAAAAAAA6vmUlBQyipOVrJSqAYygZyCxKLi1CI4HyhSXJlXklgBFFFKTc5NLE4uyiwoUdKByWYVg6TSEnOKU8FCtVwgXAsAhP6ktVkAAAA%3D

SWC Info output

Operating System:
Platform: win32
Arch: x64
Machine Type: x86_64
Version: Windows 11 Home
CPU: (16 cores)
Models: AMD Ryzen 7 3700X 8-Core Processor

Binaries:
    Node: 22.12.0
    npm: N/A
    Yarn: N/A
    pnpm: N/A

Relevant Packages:
    @swc/core: 1.11.29
    @swc/helpers: 0.5.17
    @swc/types: 0.1.21
    typescript: 5.7.2
    next: 15.2.4

SWC Config:
    output: N/A
    .swcrc path: N/A

Next.js info:
    output: N/A

Expected behavior

A change in source files should not yield duplicate code in the output file.

Actual behavior

If source file simple.js is at first

console.log("Hello, world!!");

Then

npx swc src/app/v1/simple.js --out-file public/v1/output.js --watch

Results (properly) in output.js

console.log("Hello, world!!");

But after modifying simple.js (or even just hitting save without any modification)

console.log("Hello, world 2!!");

Output.js now is

console.log("Hello, world!!");
console.log("Hello, world 2!!");

Version

1.11.29

Additional context

No response