Maybe not. Some research has shown X-rays aren't entirely accurate in showing pseudoarthrosis. Pseudoarthrosis refers to a "false joint" where the motion is taking place.
Patients with pseudoarthrosis on X-ray don't always show any actual loss of stability when the doctor opens up the spine to repeat the fusion. If you aren't having any symptoms, you may want to adopt a "wait-and-see" approach.
See your doctor at regular intervals, and see what happens in six months to two year's time. You can always have that second operation, but waiting is a good option if it turns out you don't really need it.