Users appearing twice

Hello,
Once in a while, I have a user that appears twice in the list, and stays duplicated for ever.
I found a potential explanation (at least I found a way to fix a duplicated user):
For a given user_id in the wp_usermeta table, there are two wpwhosonline_timestamp entries. Deleting one of them removes the duplication in the list.
Hopes this helps people having the same issue, and perhaps gives a hint at how to prevent it from happening at a development level.
Regards.