I was recently working with another member on "how to find sources", we were working her branches in the 1800s. Most of what she had already entered was "according to family history". Once we had about 5 profiles sourced, I noticed a duplicate profile. The duplicate had not shown up before because of the spelling of the first name and the birth was not a very good "estimate". Once I found the one duplicate, I started to realize that almost everything we were working on was a duplicate. As it turned out, a cousin was already a member of WikiTree and had entered all those family members a long time ago. The member I was helping had spent about a month entering profiles for people who already existed on WikiTree. Now, while we know that this happens a lot with Pre-1700 profiles, this was the first time I had seen it. If she had just had some "real" sources before entering the profiles, she might have discovered the duplication earlier.