question on assert and retract

[bruno patin]

is it normal that two identic call to assertz will have a positive 
answer ? for example :

assertz(toto(2)).
assertz(toto(2)).

toto(X).
X=2
yes
X=2
no

retract(toto(2)).

toto(X).
no


I had assumed that the second assertz would have failed ...