No, you just add the condition that should inhibit the insertion of a new sample:
rule "Set value of LeftArm fluent"
when
$ev: Start()
$fl:LeftArm()
not Sample( fluent == $fl )
then
Sample s = new Sample();
s.setFluent($fl);
s.setValue(-1.0);
insert(s);
end
After modification of the Q If you already have a Sample fact and need to setValue to -1.0 for the combination Start/LeftArm you'll need two rules, but you can use extends:
rule "StartLeftArm"
when
$ev: Start()
$fl:LeftArm()
then
end
rule "create Sample" extends "StartLeftArm"
when
not Sample( fluent == $fl )
then
Sample s = new Sample();
s.setFluent( $fl );
insert( s );
end
rule "set Sample Value" extends "StartLeftArm"
when
$s: Sample( fluent == $fl, value != -1.0 )
then
modify( $s ){ setValue( -1.0 ) }
end