This is a quick program that can be used to identify users based on two or more posts from their favorites.

Identify_from_favorites.js

const request = require('request');

async function find_users(id){
	const options = {
		url: `https://e621.net/favorite/list_users.json?id=${id}`,
		headers: {
			'User-Agent': 'Identify users from favorites/v1.0'
		}
	};
	return new Promise((resolve, reject) => {
		request(options, (err, headers, response) => {
			if(err || headers.statusCode != 200){
				reject();
			} else {
				const r = JSON.parse(response)
					.favorited_users
					.split(',');
				resolve(new Set(r));
			}
		});
	});
}

function set_intersection(a, b){
	return new Set([...a].filter(e => b.has(e)));
}

async function main(){
	const ids = process.argv.slice(2).map(e => parseInt(e, 10));
	if(ids.length == 0){
		return console.log('No post_ids given');
	}

	let all_users = await find_users(ids[0]);
	for(const post_id of ids.slice(1)){
		const new_users = await find_users(post_id);
		all_users = set_intersection(all_users, new_users);
	}
	console.log(all_users);
	return all_users;
}

main();