C :

#include<stdio.h>
#include<string.h>
main()
{
	char a[100],b[100];
	int c[150]={0};
	int i;
	gets(a);
	gets(b);
	strcat(a,b);
    for(i=0;i<strlen(a);i++)
    {
    	int k=a[i];
    	if(c[k]==0)
    	{
    		printf("%c",a[i]);
    		c[k]++;
		}
	}
}

C++ :

#include<iostream>      
#include<string>
using namespace std;  
int main(){           
   string s1,s2;
   int a[26];
   cin>>s1;
   cin>>s2;
   s1 = s1 + s2;
   for(int i = 0;i <= 25;i++)  
   {
   	  a[i] = 0;
   }
   for(int i = 0;i < s1.length();i++)
   {
   	  if(a[s1[i] - 97] == 0){
   	     cout<<s1[i];	
   	     a[s1[i] - 97] = 1;
	  }
   }
   cout<<endl;
} 

Python :

a = input()
b = input()
c = a + b
li=[]
for i in range (0,25):
    li.append(0)
for i in c:
    li[ord(i)-97] += 1 
    if li[ord(i)-97] == 1:
        print(i,end='')