I think the only problem people have with duo accounts is “gym blocking,” which... I don’t really understand, and is it even a thing anymore?
I think it’s fine to have a second account. If you’re already at close to max level and find yourself mostly bored with Pogo, duoing accounts can make the game fun again. There’s so much to do in the early game now compared to 2 years ago. It doesn’t really take as long to level up these days either, thanks to friends and raids. I started a second account almost 3 months ago, and it’s closing in on level 33.
Having a second account for trading is great during community day. I got 7 lucky trades from the Bagon day. I have more than enough Sinnoh stones thanks to daily PVPs. Having a Ultra friend and eventually a best friend gets you more balls in raids too.
Really the funnest thing has been building teams to duo T4 raids. Once you get your second account high enough to raid, start doing Machamp raids and build a fight squad. Once your fight squad is strong enough, start taking down Tyranitars, and build a dark team to work towards Deoxys and the Lake Trio.