vue.config.js customize extensions no priority，no Array deduplication

[AllenChinese]

Version

3.5.0

Environment info

System:
    OS: macOS High Sierra 10.13.6
    CPU: (4) x64 Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
  Binaries:
    Node: 8.11.2 - /usr/local/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 5.6.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 78.0.3904.108
    Firefox: 59.0.2
    Safari: 12.1.2
  npmPackages:
  
  npmGlobalPackages:
    @vue/cli: 3.5.5

Steps to reproduce

configureWebpack: {
resolve: {
extensions: ['.py', '.jsx', '.ts', '.tsx', '.js', '.vue', '.json'],
},
}

What is expected?

extensions: ['.py', '.jsx', '.ts', '.tsx', '.js', '.vue', '.json', '.mjs', '.wasm'],

What is actually happening?

extensions: [ '.mjs', '.js', '.jsx', '.vue', '.json', '.wasm', '.py', '.jsx', '.ts', '.tsx', '.js', '.vue', '.json' ],