While loop causing function to return 0 in c

I have the following c function working on implementing a rejection sampler. MersenneTwiser.h generates a random number between 0 and 1. The issue I'm having is with the rnorm function. I'm using a while loop to reject some samples. It's nonsense at the moment as it's not finished. In it's current form, the program returns 0.00000.

#include<stdio.h>
#include<math.h>
#include"MersenneTwister.h"
MTRand rng;

double p;
double prop;
int sign;


double dubexp(double theta){
    p = rng.rand();

    if ((p-0.5) > 0) sign= 1;
    if ((p-0.5) < 0) sign= -1;

    prop = -(1/theta)*sign*log(1-2*abs(p-0.5));
    return(prop);
}


double u;
double theta;
double t;
double z;
double c;
double x;

double rnorm(double theta, double c){

    t=rng.rand();
    while (z == t)
    {
        x=dubexp(theta);
        u=rng.rand();
        z=x;

    }
    return z;
}

int main(){
    theta=1;
    c=1;
    
    u = rnorm(theta,c);
    printf("%f",u);
}

However if I remove the while loop, it returns the correct value of z. As below:

double rnorm(double theta, double c){

    t=rng.rand();
     x=dubexp(theta);
        u=rng.rand();
        z=x;
    
    
    return z;
}