How do I know one file is during downloading?

## 💬 Questions and Help

For questions or help we recommend checking:
If I am starting a downloading progress by await DefaultCacheManager().downloadFile(url);, and if I use "await DefaultCacheManager().downloadFile(url);" again, how do I know the file is in downloading progress and not download it twice or restart the downloading?

- The [Flutter tag in Stack Overflow](https://stackoverflow.com/questions/tagged/flutter)

Start with the DefaultCacheManager().downloadFile(url) entry point named in the question and inspect the current implementation and documentation for concurrent downloads. Determine whether a second call is deduplicated or restarts the transfer; the issue provides no named test, file, or specific code change to verify.

索引モデルが issue の本文から書いたものです。