Repeating data frames in OFDM

This is with reference to the white paper by TI entitled, "Power Line
Modem with E-meter Platform", 2004 (i'm sorry i can't find the link to
the article)

At the part where the paper decribes the Packet Format for the OFDM
frames. It says that the data frames are grouped into blocks of 3
frames and each data frame in the block contains identical data but
transmitted at different frequency.

Is this a common practice to transmit identical data for multiple times
in OFDM or just a choice of transmission methods?

Q2:
The preamble and synchronization data. Is it any binary sequence will
do as long as the receiver know what is it? or is there a specific
binary sequence for those?