#include<stdio.h>
#define changdu 80
void main()
{
    char ch[changdu];
    int num,b,c,e;
    gets(ch);
    lddp: e=1;
    for (num=0;num<(changdu);num++)
    {    
        for (b=e;b<(changdu);b++)
        {
            if (ch[b]=='\0')
                goto loop;
            else
                if (ch[num]==ch[b])
                    {
                        for(c=b;c<(changdu-1);c++)
                            ch[c]=ch[c+1];
                        goto lddp;
                    }    
        }
        loop:e=e+1;    
    }
    puts(ch);
}

第一，删除字符以后字符串长度要调整，或者末尾置0
第二，goto现代编程尽量避免。

#include<stdio.h>
#define changdu 80
void main()
{
    unsigned char ch[changdu], c[256]={0};
    int num,b;
    for (num=0; num<changdu-1&&(ch[num]=getchar())!='\n'; num++) NULL;
    ch[num] = '\0';
    num = 0;
    for (b=0; ch[b]; ++b)
    {
        if (!c[ch[b]])
        {
            ch[num] = ch[b];
            c[ch[b]] = 1;
            ++num;
        }
    }
    ch[num] = '\0';
    puts(ch);
}

请参考：
https://bbs.bccn.net/thread-486798-1-1.html