[ns] hello, error please help??

[Batbold Toiruul]

Hello everybody,
I am studying NS-2 and working on examples of Marc Greis.
http://www.isi.edu/nsnam/ns/tutorial/index.html

i completed all the guidance on the section 7, however i received some
errors when i recompile ns using 'make' command.
Errors:
ping.o(.text+0x10a):ping.cc: multiple definition of
'PingAgent::PingAgent[in-charge]()'
apps/ping.o(.text+0xf6):ping.cc: first defined here
collect2: id returned 1 exit status
make:*** [ns] Error 1

please, if anyone know about this error, help me?

sincerely,
   Bold