int checkWord(const char * w)
{
int cnt[256] = { 0 };
while(*w) cnt[(unsigned char)(*w++)]++;
for(int i = 0; i < 256; ++i)
if (cnt[i] == 1) return 0;
return 1;
}

int main(int argc, const char * argv[])
{
char s[256];
char * words[50];
int idx = 0;
puts("Введите последовательность символов");
fgets( s, sizeof( s ) - 1, stdin ) )
for(char * w = strtok(s," ."); w; w = strtok(NULL," ."))
{
if (checkWord(w)) words[idx++] = w;
}
—idx;
for(int i = 0; i < idx-1; ++i)
{
if (strcmp(words[i],words[idx])) printf("%s\n",words[i]);
}
}