That's nonsense. There were two sister ships, the Britannic and the Olympic, and they are fully accounted for. None were identical. Materials taken from the wreck of the Titanic bear a serial number for that ship. There are other detail differences in construction between the Titanic and its sister ships that were identified with the wreck. There are any number of debunk sites in the internet that cover this. One site in particular goes into detail, but I was unable to find it for citation.