[BUG] Spurious duplicate files created at the end of large split archives.

### Configuration
- Keka version: 1.2.50 (4808)
- macOS version: 11.6.2 (20G314)

### Describe the bug
A couple of times, when creating large 7z split archives, the final split was be duplicated, i.e. "foo.7z.xxx" and "foo.7z 2.xxx"
MD5 indicates the files are identical.

Attach a log file reproducing the issue: As this has only occured twice on very, very large archive creation, I don't have one yet, but I'm working on it.

### To Reproduce
Steps to reproduce the behavior:
1. Create a 7z split archive. For me, it happened on very large archives that take a long time to create, but I have been unable to recreate the error using small, quickly created split archives.
2. Wait...
3. Open the folder where the archive is and you may see that the final split volume is duplicated, "foo.7z.xxx" and foo 2.7z.xxx"
4. The files are identical according to MD5 hashing.

### Expected behavior
Not have 2 files that are identical at the end of the archiving process. I don't know which final split file is the one associated with the archive, so I save both. Testing with 7zz indicates that the archive is valid with no errors, but still not sure which final file is being used.

### Screenshots

### Additional context
Can't reproduce this at the moment, but it's happened twice. I'll work on getting one done.