Walter thank you for kind words about Tonfotos!
It is really hard to say from the distance what was the reason for that, but it is pretty easy to fix:
1) Make sure both people have exact same name (just rename one of them). Renaming is available in context menu (right click on the person in the list).
2) Now you can merge them - this option will appear in the same context menu if there is more than one person with exactly the same name.
That should solve the issue.