Number of characters

Few modifications should do the job.

#include<stdio.h>
int main()
{
	char a[50],ch;
	int i,j,n=0;
	printf("enter your string :\t");
	gets(a);
	for(i=0;a[i]!='\0';i++){
                n=0; //reset the count for next character in queue
		for(j=0;a[j]!='\0';j++){
			if(a[i]==a[j])
			n++;
		}
	   printf(" %c : %d \n",i,n);//show i instead of j
        }	
}

You'll still be seeing duplicate lines as you are iterating for each characters in the string without checking for duplicates. To overcome this, you can apply logic to have a string with unique characters and that you need to use for outer loop (i).

Check following links for help regarding that-
C program to remove all repeated characters in a string - Codeforwin[^]
Remove all duplicates from a given string - GeeksforGeeks[^]

Hop, it helps :)