C'de döngü sorunu

Bu kodun main metodunda for döngüsünün 2. dönüşünde problem çıkıyor ve "Enter your choice" cümlesini 2 defa basıyor.Sebebi ne olabilir acaba?

 #include <stdio.h>
    #include <stdlib.h>

    int factorial(int n);
     void prime_numbers(){

        int upper_bound;
        printf("Define upper bound:");
        scanf("%d",&upper_bound);
        printf("Prime Numbers: ");

        int i,j;
        int variable;
         for (i=2;i<=upper_bound;i++)
            {
              variable = 1;
                for (j = 2; j <i; j++)
                   { if (i % j == 0)
                        {
                            variable = 0;
                            break;
                        }
                   }
                     if (variable == 1)
                        printf ("%d ", i );
            }
    }

     void leibniz_series(){

        printf("Define k value:");
        int k;
        scanf("%d",&k);
        double sum=0;

        int i;
         for (i=1; i<k; i++)
        {
                if ((i%2)==1)
                    sum=sum+1.0/((2.0 * (double)i) - 1.0);
               else
                sum = sum - 1.0/((2.0 * (double)i) - 1.0);
            }

        printf("Result:%f\\n",4*sum);
    }

     void combination(){

        int comb;
        printf("Define n and r:");
        int n,r;
        scanf("%d %d",&n,&r);
        comb=factorial(n)/(factorial(r)*factorial(n-r));
        printf("Result:%d",comb);

    }

    int factorial(int n){
        int f=1;
        int i;
        for(i=1;i<=n;i++)
        f=f*i;
        return f;
    }

    int main()
    {
        printf("Press P or p for Prime Numbers\\n");
        printf("Press L or l for Leibniz Series\\n");
        printf("Press C or c for Combinations\\n");
        printf("Press E or e for Exit\\n");

       for(;;){
        char choice;
        printf("Enter your choice:");
        scanf("%c",&choice);

        if(choice=='E' || choice=='e')
            exit(0);

        switch(choice){
            case 'p' :  prime_numbers();
                            break;
            case 'P' :  prime_numbers();
                            break;
            case 'l' :  leibniz_series();
                            break;
            case 'L' :  leibniz_series();
                            break;
            case 'c' :  combination();
                            break;
            case 'C' :  combination();
                            break;
           // default  :   printf("Wrong choice\\n");

        }

    }
        return 0;
    }