Writing non-duplicates

[C-Struggler]

I was wondering if someone could give me some assistance with my program. What I have right now is a file called 'duplicates.txt' which contains the following integers I maually entered:
1
1
2
3
3
3
4
5

I can read the file and write the file, the problem is I need to only write one of each number to the written file called 'non-duplicates.txt'. I don't know where to start with this. Here is the code that I currently have. Any help would be appreciated.

#include <stdio.h>

int main(void)
{
	FILE *file;
	char buffer[10];

	file = fopen("duplicates.txt", "r");
	
	while (fgets(buffer, sizeof(buffer), file))
		printf("%s", buffer);
	return(0);
	fclose(file);

	file = fopen("non-duplicates.txt", "w");
	fwrite(buffer, sizeof(buffer), 10, file);	
	fclose(file);
  
}

[Hammer]

You need to *remember* the numbers as you read them. Various methods are available including binary trees and arrays. Binary trees are more robust but are more complex to code... arrays are simpler, but are of fixed size and can be slow to search through. As you're a beginner, I'd suggest sticking with array for now though...