请问这里的printf为什么会输出两遍？应该怎么解决？

问题相关代码，请勿粘贴截图

#include<stdio.h>
#define a0 2.05
#define b0 1.15
#define c0 1.09
int main(void)
{
    double pound=0;
    double sum=0;
    double price=0;
    char ch='0';
    while(ch!='q'){
        printf("which are you want?\n");
        scanf("%c",&ch);
        switch(ch)
        {
            case 'a':
                printf("how much you want?\n");
                scanf("%lf",&pound);
                price=price+pound*a0;
                sum=sum+pound;
                break;
            case 'b':
                printf("how much you want?\n");
                scanf("%lf",&pound);
                price=price+pound*b0;
                sum=sum+pound;
                break;
            case 'c':
                printf("how much you want?\n");
                scanf("%lf",&pound);
                price=price+pound*c0;
                sum=sum+pound;
                break;
            default: break;
        }
    }
    
    if(sum<5){
        price=price+6.5;
    }else if(sum>5&&sum<20){
        price=price+14;
    }else{
        price=price+(0.5*(sum-20)+14);
    }
    
    if(price>100){
        price=price*0.95;
    }
    printf("%lf %lf",sum,price);
    return 0;
}

运行结果及报错内容

我想要达到的结果

which are you want 只输出一遍

每个scanf("%lf",&pound);语句后添加一个getchar();来忽略后面的回车