M39/B can spawn with double flashlight causing a fix-me sprite

### Testmerges

N/A

### Round ID

N/A

### Description of the bug

Specifically the under barrel rail light and the normal rail light
Which would be fine if it didn't result in:

### What's the difference with what should have happened?

Uhm not a random B8 sprite and a Fix me Icon

### How do we reproduce this bug?

1. Roll the dice
2. Spawn with an M39/B
3. Which spawns with a rail light and a rail light but underbarrel'd
4. Panic?! The horror
...

### Issue Bingo

- [x] Issue could be reproduced at least once
- [x] Issue happened in a recent (less than 7 days ago) round
- [x] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)

Search the M39/B spawn or equipment definition and trace how its rail-light attachments are assigned. Reproduce the listed spawn sequence, then verify that the M39/B produces only the intended light and no fix-me sprite.

Written by the indexing model from the issue text.