This is one possible formula option (:B is group column)
New column("Peak", Numeric, Nominal, Formula(
prev_val = Col Moving Average(:Length, 1, 1, 0, :B) * 2 - :Length;
next_val = Col Moving Average(:Length, 1, 0, 1, :B) * 2 - :Length;
:Length - prev_val > 0 & :Length - next_val > 0
));
-Jarmo