There are a couple of reasons why your iPhone Duo won't see a connected external drive, but more importantly, there's a solution.
So you have an external drive that plugs into the USB-C port of your old iPhone perfectly fine. But when you connect it to your iPhone Duo, nothing happens.
Not to build up suspense here, there are two possible solutions. One is that if you have the iPhone Duo in a case, take it out and try again.
The other is to change the USB-C cable.
You will typically have had an extension cable when you bought the drive. The odds are that this will do the job, and so would other USB-C cables.
In AppleInsider testing, the problem came up with a Lexar drive that had a USB-C connector built in. The drive came with an extra cable that was intended for camera connections, and that worked.
Why this happens
Clearly if you have a case, it's not surprising if a USB-C connector isn't long enough to reach through to the iPhone. But if the drive did work through the case of that other iPhone, you would expect it to work through a similar case on the iPhone Duo.
Left: the Lexar drive can't be seen when connected directly. Right: use Lexar's provided extension unit for use with cameras and it works perfectly.
The problem is definitely due to the length of the USB-C plug and the depth of the USB-C socket. There's no information from Apple about the iPhone Duo being any different to previous recent iPhones on this issue, but it is.
Equally, there is currently no information from case manufacturers on this point, and there probably won't be. But the extension cable trick will work with any of them.
It's definitely less convenient having to carry an extra cable as well as an external drive. But then the fact that you can do it means you could save a lot of money and buy an iPhone Duo with less storage than you're used to.