Invalid composer.lock when the same requires-dev dependency is present on more than once composer.json

[marcoslois]

Given two composer.json files, root/composer.json:

{
  "name": "marcos/merget-test",
  "require": {
    "wikimedia/composer-merge-plugin": "~1.3.0"
  },
  "require-dev": {
    "phpunit/phpunit": "^4.8"
  },
  "extra": {
    "merge-plugin": {
      "include": [
        "mods/*/composer.json"
      ]
    }
  }
}

root/mods/test/composer.json:

{
  "name": "marcos/mod-test",
  "require": {
    "monolog/monolog": "^1.19"
  },
  "require-dev": {
    "phpunit/phpunit": "^4.8"
  }
}

When running composer update

Then the composer.lock need to have packages and packages-dev populated.

Error the composer.lock have a packages-dev empty, this causes that development packages get installed when running composer update --no-dev. All the require-dev dependencies are merged, but included in packages instead of packages-dev.

If the requre-dev is only present in one composer.json runs the expected behaviour.