Groupees duplicate keys ?

It can happen on any site, including Humble or any non-bundle site as well.
Groupees provides support for anything as long as they are in contact with the publisher. If the contract ended, it is up to the publisher whether they replace or not, but most one-man indie "teams" do that.

Twice I've gotten duplicate keys from them and both times support sent me a new key in couple of hours.

Both were from newly released bundles.

They also claimed groupees support is not provided for things 6+ months old.

Nope, that's not true. Tell them to contact support and get a new key.

Groupees does have issues with duplicate keys sometimes when a large amount of traffic is going around (generally when new bundles launch). That being said this sounds like somebody who forgot they activated a key or is intentionally using an activated key and trying to scam you. Telling you that you can't contact Groupees support past 6 months is just further misdirection. They (Groupees) don't just not honor unactivated keys after 6 months. They said that after 6 months if the key has an issue (such as being a dupe) it gets harder to replace them due to license agreements.

If you traded the key you were duped by the user. I had a bad trade once giving away 3 keys fresh for Gunslinger Juarez key. There is no guarantee the key you get is unused. The trader would not reply and kept my friend request deliberately hanging for months. Groupees support finally gave me a replacement key after I pestered support to take action against that guy. Support is co-operative. But trading games is a huge gamble there.

From my experience there does seem to be an issue with dupe keys on Groupees (especially old ones). I've had a couple trades where the very same thing happened to the person I was trading with (reliable people too with rock solid rep). They provided me keys that were already used and couldn't figure out what was going on.

Do you trust groupee keys?

A lot less than I used to.