Problem Solved!! I just wrote this line:

<param name="max-registrations-per-extension" value="1"/>

within the "params" tag when defining a new user in the
~/freeswitch/conf/directory/default/ directory. Then issue "reloadxml"
command and everything works as I wanted to: if Extension A is already
registered with id ID_A and extension B tries to register with id ID_A,
the registration is rejected ^_^

Thank you!

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Dr. Villa Alberto
Sw Engineer