Compiler throws declaration_duplicate for doubled `var` declarations

[ginpei]

Describe the bug

Having two variable declarations using var is valid in JavaScript, but the compiler throws declaration_duplicate error.

I found this issue during my Svelte 5 migration. Here is a scenario:

1. Had a Svelte 4 project using $$props:

   <script lang="ts">
     const allProps = $$props;
   </script>

2. npx sv migrate svelte-5 converts the component into like:

   <script lang="ts">
     interface Props {
       [key: string]: any
     }
   
     let { ...props }: Props = $props();
     const allProps = props;
   </script>

3. Build the project with esbuild-svelte

4. The error occurs. The message doesn't make sense

   ✘ [ERROR] `p` has already been declared
   https://svelte.dev/e/declaration_duplicate [plugin esbuild-svelte]
   
       src/App.svelte:7:24:
         7 │   const allProps = props;
           ╵                         ^
   …
   

Reproduction

By tidying up the source given to compile(), here is a minimal reproducible example:

const { compile } = require('svelte/compiler');

const source = `
<script lang="ts">
var oops;
var oops;
</script>
`;
compile(source, {});

Logs

[jr [CompileError]: `oops` has already been declared
https://svelte.dev/e/declaration_duplicate] {
  code: 'declaration_duplicate',
  filename: '(unknown)',
  position: [ 34, 38 ],
  start: { line: 4, column: 4, character: 34 },
  end: { line: 4, column: 8, character: 38 },
  frame: '2: <script lang="ts">\n' +
    '3: var oops;\n' +
    '4: var oops;\n' +
    '           ^\n' +
    '5: </script>\n' +
    '6: '
}

System Info

  System:
    OS: Linux 5.15 Ubuntu 20.04.6 LTS (Focal Fossa)
    CPU: (16) x64 AMD Ryzen 7 5800H with Radeon Graphics
    Memory: 2.85 GB / 7.46 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 22.6.0 - ~/.nvm/versions/node/v22.6.0/bin/node
    npm: 10.8.2 - ~/.nvm/versions/node/v22.6.0/bin/npm
  npmPackages:
    svelte: ^5.0.0 => 5.20.2 

Severity

annoyance

[ginpei]

I'm not sure if this is supposed to be marked as upgrade blocker because you can avoid the build error by replacing let { ...props } = $props() part with let allProps = $$props