Hi Sven,
I was able to test your naming format for the Persons Shown field on both my Mac and PC test machines. When I use the Filter Tab -> IPTC Fields -> Persons Shown, I am not able to recreate the issue in your screen shot. While testing your configuration, I did notice a few duplicate entries for last names with an apostrophe. After closer inspection, the apostrophe characters were actually different. The solution was to use the Find and Replace utility. The Find field was set to use the name from one set of images with the fewest number. The Replace field was using the name from the other set with the most number of images. Once the operation was done, both sets were combined together.
I'm wondering if the same trick might work for your copy of Photo Mechanic. Click the entry in the Filter panel with just the last name. Open the IPTC Info panel for any of the images in this contact sheet. Copy the last name and any trailing text up to the next semi-colon separator, if there is one present. Paste this into the Find and Replace utility's Find field. Next click the matching Last name, First name entry in the Filter panel. Open an IPTC Info dialog and copy the correct Last name, First name from the Persons Shown field. Paste this into the Replace field in the Find and Replace dialog. Go back to the first contact sheet with just the Last name shown for the Persons Shown. Now use the Find and Replace utility, just updating the Persons Shown field. I hope this helps.
Best regards,
--Bob