Good Day
I have also experienced an issue adding a .csv file to a map. In my case i exported water quality monitoring point data using a standard format. I broke the data exported into different catagories namely River, Water Body, Discharge Accidents, Water Use Sites and Transfer features. I then imported most of the files into ArcGIS online, however the Transfer feature data file gave me problems. First there was an error that stated that there was no coordinate data, then if i made any change to the transfer feature .csv i then got an error stating that the file could not be added as several fields were duplicates. I eventually figured out that ArcGIS online was reading the Column headers as duplicate fields, once i modified the headers then the file imported correctly. Does anyone have an idea why i would only get this problem for the one import (remember all the .csv's had the same format and column headers).
Brett