grunt-contrib-copy stops with EEXIST error

Question

I am using grunt-contrib-copy to create a build/ folder for my NodeJS express application before deploying it. When running the command, it fails with the error that the folder already exists (Error code: EEXIST).

The copy block in the Gruntfile looks like this:

copy : {
  build : {
    options : {
      mode : true
    },
    files : [
      {
        expand  : true,
        src     : files_to_deploy,
        dest    : 'build/',
        flatten : false,
        filter  : 'isFile'
      },
    ],
  },
},

The files_to_deploy is an array containing the files. In a shortened form, it looks like this:

const files_to_deploy = [
  'app.js',
  'package.json',
  'controllers/**',
  'models/**',
  'routes/**',
  '!**/*.test.js'
];

When I am calling grunt copy:build --verbose, I get the following output (shortened).

[...]
Running "copy:build" (copy) task
Verifying property copy.build exists in config...OK
Files: app.js -> build/app.js
Files: package.json -> build/package.json
[...]
Options: encoding="utf8", processContent=false, processContentExclude=[], timestamp=false, mode
Copying app.js -> build/app.js
Reading app.js...OK
Writing build/app.js...Warning: Unable to create directory "build" (Error code: EEXIST). Use --force to continue.

Aborted due to warnings.

The thing is, grunt creates the build/ folder. And it even successfully copies the full app.js file into it. But it then simply stops.

I checked permissions and they look good. I tried to run it with sudo and it fails with the same message. I checked everything out into a fresh folder, same error. And I even have a very similar project with basically an identical structure and Gruntfile and everything works there.

Additional details:

• node version: 14.15.1
• grunt: 1.6.1
• grunt-contrib-copy: 1.0.0
• WSL 1

Any help, hints or points are highly appreciated!