It is not clear which family tree program/data base you are referring to, but unfortunately, this is a common issue with online genealogy databases, as most programs allow easy copy of information from one tree to another. This is the downside of online databases - anyone can copy them, edit them, add them to their own family tree and claim ownership. There is no control and no research or documented sources needed and most of these databases/websites don't even have the option of showing the source of the information. Besides the personal frustration of the original researcher, it also causes errors to be copied again and again, until there is no way to tell correct information from wrong information. I find it all the time with my family tree which I uploaded to FTJP more than 20 years ago. From there is was copied to Geni.com and has been copied by others into their family trees in other genealogy databases countless times. When my database (MyHeritage) informs me of data match, I can tell , in most cases, that those trees are based on my original tree because like in Arline's case, there are pieces of information that only I would know. I even recognize errors that I previously made in my research I have since corrected in my own database.
This is the main reason I decided not to upload my complete tree anymore. I have a basic tree in MyHeritage that goes back a few generations, for the purpose of connecting to relatives, but I did not upload the full tree, which goes back to the 11th century and took me more than 30 years to research, including countless hours at the library, prior to the internet . I gladly share and compare information with family members and other people on a personal contact basis, but for all the reason I listed above, I would not upload my entire tree for everyone to copy and claim as their own work.
Don't get me wrong - I do use information I find in online trees in my family research, but I only do that after carefully considering the information, comparing it to other information I have, reviewing the source (if available) and with detailed notes about the source and it's reliability. In most cases, I try to contact the person who posted the information to clarify the source(s) of that information before I use it.
Cherry Hill, NJ, USA (originally from Israel)
Researching: Israelite (from Novogrugok in Belarus and Latvia), Rabinowitz (Dyatlovo/Zhetl, in Belarus), Pruss (from Ukraine), Koifman from Ukraine and Goldblat (Ukmerge/Wilkomir., in Lithuania)