I had more info from my colleagues in Tokyo. Apparently this happened when they opened the file as an attachment through their email client (Outlook) but did not happen when saving the file to their desktop first.

Additionally, if they adjusted the column width; all those duplicate words just disappeared!

I cannot reproduce this error on my end when opening directly through my email client (Thunderbird) so it's not really possible to send a before and after slide as they'll both just look as intended.

Oh well.. it's never happened before and may not happen again so I guess it's nothing to worry about but it is an oddity!