The scenario you describe is possible. It's the result of user error, but it would probably be a very subtle user error of some sort - the sort of thing where you wouldn't even notice the error.
For example, at some point in your processing you might enter a Place name. At another point in your processing - perhaps hours or days or months later - you might enter the same Place name but spelled ever so slightly differently. Perhaps the difference is a difference in spacing or capitalization or almost anything trivial - e.g., "Great Falls, Montana" without the quotes vs. "Great Falls, Montana" without the quotes. Notice that the second instance of the name has an extra blank that doesn't belong. At yet another point in your processing years later, you might notice in the Edit Person screen that one or the other of the place names is spelled wrongly somehow or other and you correct it. At this point, you will have two identical place names in your database. They will need to be merged in your Place List. It would have been easier and better to have fixed the problem with a merge in the Place List in the first place, but that's not where you noticed the problem. You noticed the problem in the Edit Person screen.
Now suppose that in addition to the above, you have some burials at the Shady Rest Cemetery in Great Falls which you entered into the Place Details field, spelled exactly the same in all instances. Except that most of the burials in the Shady Rest Cemetery were recorded in "Great Falls, Montana" without out the quotes and one of the burials was recorded in "Great Falls, Montana" without the quotes. After merging the two instances of Great Falls, Montana in the Place list to get rid of the duplication and the name with the double blank, you will be left with a duplicate instance of Shady Rest Cemetery in the Place Details list under the Place name item for Great Falls, Montana. So you have to do the same sort of merge in the Place Details list that you did in the Place list.
It's worth going through the Place list from time to time, eyeballing for funny looking or duplicate entries. There is no automated procedure for this, but problems usually stand out like a sore thumb. Fix them with a merge. And after the Place List merge, then eyeball the Place Detail list for the Place that you just merged.