#include <iostream>
#include <set>
#include <algorithm>
 
using namespace std;
 
int main() {
 
	std::set<int> ints = {1, -1, 3};
 
	auto dupe = std::find_if(ints.begin(),ints.end(),[&](const int& first){
			return std::find_if(ints.begin(),ints.end(),[&](const int& second) {
				// so a the same value isn't checked against itself..
				if (&first == &second) return false;
				return std::abs(first) == std::abs(second);
 			}) != ints.end();
		 } );
 
	if (dupe != ints.end()) std::cout << "Found dupe: " << *dupe << std::endl;
}

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