Take this stuff with a grain of salt. ...maybe even a whole salt shaker. Usually when you investigate such things you'll find that they are grossly exaggerated or simply not true. There are also numerous hoaxes played on the scientific establishment.

There's little doubt that we don't really know nearly as much as we think we do and there is some data that scientists sometimes just prefer to ignore.

It's not impossible that there was another species that was similar to man that had developed some technology in the past but I'm aware of no solid basis for believing so. Every indication is that our DNA is so ancient that it preceded life on Earth. There's no reason a species might not arise multiple times if a niche opened and closed multiple times. Of course, each of these species would be distinct no matter how similar. The planet has been here for 5 billion years and probably has supported life for at least 3 billion. This is plenty of time for many species to come and go. Bear in mind, however, that an advanced technology and high population like we humans would leave so many fossils as to be impossible to overlook.