Metadata not updating when using discogs plugin

### Problem

Beets doesn't appear to be updating durations. This means that it is attempeting to update the files over and over again. I have updated the read right so user/group/others have write access but the same issue still occurs. I can confirm that it does update other fields like Artist.

Looking at the songs in my music players, I am not convinced that the duration tag is meant to be writable. Kind of feels like this information should be part of the mp3 properties already. Is beet mean to be overwriting the duration metadata? if not, how do I make is so that it ignores this metadata and doesn't attempt to update the files to fix it?

As you can see below, the while beets finds that the duration should be updated, the duration is the same before and after import:


### Setup

* OS: Ubuntu 23.10
* Python version: 3.11.6
* beets version: 1.6.0
* Turning off plugins made problem go away (yes/no): n/a

My configuration (output of `beet config`) is:

```yaml
directory: ~/Music
library: ~/.config/beets/musiclibrary.db

pluginpath:
~/.config/beets/plugins

import:
move: yes
copy: no
write: yes

#set threshold for auto matching (0.10 = 90% match)
match:
strong_rec_thresh: 0.15
```

Reproduce the repeated duration update during an import with the reported Ubuntu, Python, beets, and configuration details, first checking whether the Discogs plugin is involved. Trace the import and metadata-writing path to compare duration before and after import; done means the cause is identified and repeated unnecessary updates no longer occur.

Written by the indexing model from the issue text.