After researching my tree for the last 25 years, I make sure that it has never been posted on any internet site. If I see that someone else is researching one of the lines, I will contact them, and if they seem genuine, will share information if it is of help to them.
Even so, you cannot guarantee they will not make it public, and you cannot guarantee they will post the correct details.
After many years of research, it does gripe when someone can cribb your work from the net in a few hours, and then claim it was "all their own work".

Steve