When you add a field to a contenttype / list through the UI, the title of the field becomes the internal name. Thus, when you add two fields with title = “Last Name”, adding the last one results in an error.
However, sometimes you want to have 2 fields with the same title. Of course it is possible through a feature, but it is also possible through the UI.
Let’s say you want to add two fields with title = “City”. First add a field named “City”, then add a field named “City2”. Rename the last field after adding to “City”, et voila, it works. The internal name of the second field stays “City2”, but the title has changed to “City”.
Very simple solution, but worth mentioning.