Prevent shy space from being added multiple times

I sometimes use JoliTypo to fix a text which has already been fixed before. If my text contains `Personnalisation`, after multiple saves, as many `­` will be added, which ends up giving me this kind of content :
```
Person­­­­­­­­na­­­­­­­­li­­­­­­­­sa­­­­­­­­tion
```
A lazy solution would be to replace (with regexp) `­­` with `­`.

Todo :
- [ ] Check if this is happening with other fixers
- [ ] Check if this is du to JoliTypo or org_heigl/hyphenator