sprintf: anomalous behavior

Hi everybody,
I happened the following thing using sprintf function on different
machine :
on Digital Alpha
s1="PIPPO" and s2="PLUTO"
sprintf(s1, "%s%s", s1,s2) -> s1 is equal PIPPOPLUTO
while on HPUX
s1="PIPPO" and s2="PLUTO"
sprintf(s1, "%s%s", s1,s2) -> s1 is equal PLUTOPLUTO

somebody knows as can i get the same behavior on HPUX to?

thanks all
Ivano

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