Removing any duplicate characters in a string

I have piece of code in C which removes any duplicate characters in a string. But I am doing it in two loops and would like it to optimize it to one loop.

void removeDuplicates(char* ptr)
{
    int end = 1;
    int length = strlen(ptr);
    int current;
    int i;
    current = 1;

    for(end=1; end<length; end++)
    {
        for(i=0; i<end; i++)
        {
            if(ptr[end] == ptr[i])
            break;
        }

        if(i == end)
        {
            ptr[current] = ptr[i];
            current++;
        }
    }

    ptr[current] = 0;
}

int main(int argc, char** argv)
{
    char str[256] = {0,};
    gets(str);
    removeDuplicates(str);
    printf("%s\n", str);
    return 1;
}

Remove Duplicates: (Re-Write Ant's code so it is readable).

#include 

void removeDuplicates(char* ptr);

void removeDuplicates(char* ptr) 
{
    char seen[UCHAR_MAX+1] = {0};

                              //
                              // Smallest data type is a char (unless you want to bit
                              // twiddle, and that has been shown to be not worth it
                              // see the complaints about std::vector in C++).
                              // 256 bytes is not much.
                              // 
                              // By using char as they type it prevents all the complex
                              // multiplication/division an bit twiddling that Ant was doing.

    unsigned char* source = (unsigned char*)ptr;
    unsigned char* dest   = (unsigned char*)ptr;
    unsigned char  next;

    do
    {
        next  = *source;       // Get next character.
        if (!seen[next])       // Only enter the `if block` if we have not seen it.
        {
            seen[next] = 1;    // Mark it as seen
            *dest = next;      // Move it to destination.
                               // Note: source will change faster than dest when we
                               //       start seeing dupes, this acts as the copy back
                               //       over the duplicates.
            ++dest;
        }
        ++source;
    }
    while (next != '\0');      // Once we have copied the null terminator quit.
}

StackExchange Code Review Q#5441, answer score: 7