Build fails with route named 'index' when using vercel adapter

Describe the bug

Unable to build when an 'index' route page is present

Using @sveltejs/adapter-vercel
error during build:
Error: EEXIST: file already exists, symlink '![-]/catchall.func' -> '.vercel/output/functions/index.func'

Seems related to #13679
Works when downgrading to @sveltejs/adapter-vercel 5.6.3

Reproduction

Repro

Running npm build results in the error.

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-auto: ^3.0.0 => 3.3.1 
    @sveltejs/adapter-vercel: ^5.7.0 => 5.7.0 
    @sveltejs/kit: ^2.5.27 => 2.20.7 
    @sveltejs/vite-plugin-svelte: ^5.0.0 => 5.0.3 
    svelte: ^5 => 5.28.2 
    vite: ^6.0.0 => 6.3.3

Severity

serious, but I can work around it

Additional Information

No response

Start with the linked SvelteLab reproduction and run npm build using @sveltejs/adapter-vercel 5.7.0. Trace the adapter's route and function generation for the index and ![-]/catchall routes, then verify that the reproduction builds successfully with an index route present.