Thank you for pointing this out. I'm currently in the process of validating the data after a Mapathon and I do this in changesets: 2 people mapped the same building(neither outline was added by myself, but it shows under my name because I was the last one to significantly edit it), but I have only had time to validate the changes by one of them. I will be deleting the second outline once I get to the mapping done by that person (because they mapped outside their task), but I have to be systematic when validating to make sure I don't miss anything. Again, thank you for bringing this to my attention; by the time I finish validation, there will be no duplicate outlines.