Why does Manipulate execute the expression twice?

Question

When I execute Foobar[] in the code below and move a slider, the variable grad is printed twice. Why is this? How can I make the Manipulate body only execute once on each update?

Foobar[] := Module[{f, vars, grad},
   f = a x^3 - b x^2 + c y^2;
   vars = {a, b, c};
   With[{fman = f, controls = Apply[Sequence, {{#, 0}, -5, 5} & /@ vars]},
    Manipulate[
     g = fman/(x^2 + y^2 + 1);
     grad = D[g, {{x, y}}];
     Print[grad];
     , controls]
    ]
   ];

Edit: As noticed below, this function is creating a Manipulate. My concern is why the Manipulate appears to be executing twice for every one update. This is a minimal working example and hence there is significant amount of code (that takes a while to run) excluded from inside the Manipulate body. I included Print to emphasize that the code appears to be executing twice.

Answer 1

I think what you need is ContinuousAction -> False and TrackedSymbols -> vars:

Foobar[] :=
  Module[{f, vars, grad}, f = a x^3 - b x^2 + c y^2;
   vars = {a, b, c};
   With[{fman = f, controls = Apply[Sequence, {{#, 0}, -5, 5} & /@ vars]},
    Manipulate[
     g = fman/(x^2 + y^2 + 1);
     grad = D[g, {{x, y}}];
     Print[grad];
     , controls, ContinuousAction -> False, TrackedSymbols -> vars]
   ]];

ContinuousAction -> False is needed to keep from evaluating continuously while dragging the sliders. In the original version this happened and the Print statements were sent to the Messages window.

TrackedSymbols -> vars is needed to keep Mathematica from reevaluating when the values of g or grad change (which would be necessary if these were interdependent).