Ipfs add --copy should replace duplicate blocks

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/kubo/issues?q=is%3Aissue) for my issue.

### Description

I think this particular issue has been brought up in different contexts, (https://github.com/ipfs/kubo/issues/8948), but here's a practical example.

Folder A is added to ipfs with --nocopy
Folder B is added with --nocopy. Folder B contains blocks that are duplicated with Folder A

Later, Folder A is removed, (Upinned, Removed from MSF, and deleted from host filesystem)

The blocks that used to be duplicated by Folder A will no longer be available. (they still reference the files in Folder A)

Even a garbage collection and re-adding Folder B will not correct this issue. The only easy way to fix this, once the problem is perceived, it Unpin Folder B, run Garbage Collection, then Re-add Folder B

My proposal here is that when ipfs add --nocopy finds a dubplicate block, it should replace the reference to the new file (this could be optional if there is a case for not doing so.) That way, missing blocks could always be restored by re-adding all the content, without having to unpin and gc first.