Weird problem in SPLITMIN problem

My issue

I just submitted the answer for this problem. My approach was right but the solution failed in one testcase.
I tried to make my answer as similar to the one given in the solution to find what was wrong in my code, but I still failed at that one testcase and debugger doesn’t display that testcase.

I identified the problem but don’t quite understand it. Turns out while sorting the array I used a custom function which return a.first <= b.first (giving wrong answer to that testcase).
What it should have done is a.first < b.first (without equality).

I fail to see why that matters at all in the above question.

My code

#include <iostream>
#include <bitset>
#include <cmath>
#include <stdio.h>
#include <climits>
#include <stdio.h>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <queue>
#include <algorithm>
#include <numeric>
using namespace std;
void testcase();
bool comp(const pair<int,int> &a,const pair<int,int> &b);
int main(){
	int t; scanf("%d",&t);
	while(t--) testcase();
}
void testcase(){
	int n;;
	scanf("%d",&n);
	vector<pair<int,int>> arr;
	for(int i = 0; i < n; i++){
		int a, b;
		scanf("%d %d", &a, &b);
		arr.push_back({a,i});
		arr.push_back({b,i});
	}
	sort(arr.begin(), arr.end(), comp);
	int ans = INT_MAX;
	vector<bool> check(2*n, false);
	int i;
	int temp = 0;
	if (i<0) i = 0;
	for( i= 0; i<2*n; i++){
		temp += 1 - check[arr[i].second];
		check[arr[i].second] = true;
		if (temp != n) continue;
		if ((i-1) >= 0){
			if (arr[i].second != arr[i-1].second){
				ans = min(ans,arr[i].first - arr[i-1].first);
			}
			else{
				if ((i-2) >= 0){
					ans = min(ans,arr[i].first - arr[i-2].first);
				}
			}
		}
	}

	printf("%d\n", ans);
}



bool comp(const pair<int,int> &a, const pair<int,int> &b){

	return (a.first <= b.first);

}


Problem Link: SPLITMIN Problem - CodeChef