De-duplicate Application URL options during project conversion

### Describe the bug

I tried to convert my qbittorrentui app and was presented with duplicate options for app URL.
```
-- Application URL -----------------------------------------------------------

What website URL do you want to use for this application? The following URLs
are defined in your existing 'pyproject.toml'; select 'Other' to provide a
different URL.

1) https://github.com/rmartin16/qbittorrentui
2) https://github.com/rmartin16/qbittorrentui
3) Other

Application URL [1]:
```

### Steps to reproduce

Clone https://github.com/rmartin16/qbittorrentui/ and run `briefcase convert`

### Expected behavior

De-dup these options.

### Screenshots

_No response_

### Environment

- Operating System: any
- Python version: any
- Software versions:
- Briefcase: `0.3.21.dev120+gec9fbb4a`

### Logs

_No response_

### Additional context

_No response_

Start by reproducing the issue with `briefcase convert` against the referenced qbittorrentui project and inspect how duplicate Application URL values are read from `pyproject.toml`. Done means the conversion prompt presents each URL only once while retaining the `Other` option.

Written by the indexing model from the issue text.