[webpack] resulting bundle has duplicate css

[TheComputerM]

Describe the bug There is duplicate CSS generated for the bundle.

To Reproduce Just clone the sapper-template-webpack repo and run npm run dev or npm run export and open the devtools then click the h1 (or any other element) on index.svelte to see there is duplicate css generated.

Expected behavior No duplicate css generation

Information about your Sapper Installation:

• The output of npx envinfo --system --npmPackages svelte,sapper,rollup,webpack --binaries --browsers

System:
    OS: Windows 10 10.0.19041
    CPU: (4) x64 Intel(R) Core(TM) i5-6200U CPU @ 2.30GHz
    Memory: 2.73 GB / 7.80 GB
  Binaries:
    Node: 14.2.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.5 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.14.8 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 81.0.4044.138
    Edge: Spartan (44.19041.423.0), Chromium (85.0.564.51)
    Internet Explorer: 11.0.19041.1
  npmPackages:
    sapper: ^0.28.0 => 0.28.10
    svelte: ^3.17.3 => 3.29.0
    webpack: ^4.7.0 => 4.44.2

Severity Not much severe but fixing this can lead to better performance

Additional context ~~https://svelte-materialify.vercel.app/ was exported using webpack~~