JS47
Not ever having used Unique for duplex, I can not directly answer your question. I have however, had excellent results with both 4.7 grains of IMR 4227 or 4.7 grains SR 4759 and the remainder of the case nearly full of 1.5 Fg or 1 Fg Swiss black powder. I trickle the Swiss through a 24 inch drop tube, leaving the case just short of being full, by the thickness of my over powder wad. A card wad that is a snug fit in the case mouth of approximately .025 works well. I have found that compressing duplex loads does not increase accuracy and have experienced some nasty pressure peaks compressing Olde Eynsford duplex loads. Straight Olde Eynsford powder produces good accuracy but don't duplex it.
I have had good accuracy leaving a 1/16 inch gap between the case mouth and the bullet base. I have also had good accuracy with the same rifle by filling that gap with a dry felt wad. When the humidity is high, just about any combination is accurate. When humidity is low, things get flaky. Powder, wad combinations, bullet lube, cleaning regimen, all become critical for good accuracy. In low humidity, you really learn a lot about what it takes to make a rifle shoot well with black and duplex.
Duplex definitely shoots cleaner than straight black but is not any more, nor any less, accurate at 100-200 yards. Shooting off my double rest at 200 yards, on good days, I have shot a bunch of 1 minute of angle (plus/minus .1 inch), 10-shot groups with both duplex and straight black. Generally, range conditions are less than ideal and I feel good when my 10-shot groups run 1.5 MOA.

JS
As long as the powder has been stored dry, your old GOEX should work. When using powder that old, I suggest you pour out a small quantity of it on a plain sheet of paper. Spread it out with a finger and look at it closely to see if it  has a much in the way of powder dust (fines). Those fines cause ignition inconsistency which will frustrate your accuracy goals.
If your GOEX has fines, PM me and I'll tell you how to sift out the fines.
For the first couple of loadings, bulk measure of your powder is fine. Once your cases are well fire-formed to your chamber, I recommend you weigh your powder charges, both smokeless and black.
If you want to shoot duplex because it's fun, it is, you will have enjoyable times. If you want to shoot duplex and make nice small groups, it's as much work as any other bench rest shooting discipline. It's still fun, IMO, but a lot of effort.

Many years ago, probably at least 40, the American Rifleman had an article about the hazards of duplexing over 30%.  As I  recall the Canadian target rifle rules were close to the line.  Most US rules were in the 10% range.

John Boy,

Ever light a match to one of the socks after emptying it?  I would be way too tempted!

Jack

I have never sifted smokeless powder but what J Louis posted is absolutely true, the same with black powder. I bought a complete set of certified laboratory sieve screens from 10 to 80 mesh and all of the several brands and grades of BP have been screened that are inventory
Example, Goex Cartridge untouched in the cans has a close FFg sieve mesh ratio.  I shoot alot of it and got decent groups out to 1000 but one day decide to sock it.  Instead of the normal 45-70 charge, I dropped 2 grains.  On a 1000yd target, I had 7 out of 10 rounds in the 10 circle and center hits. Made me a believe believer socking and screening powder.  Was so enthused with the Goex CTG accuracy, when I heard it was to be discontinued - bought every can of it in my retailer's trailer
John, thanks for the advice about screening smokeless to increase accuracy too.  Thought never entered in my mind screening smokeless.  I'll screen the 5 lbs of 300-MP plus the 16 lbs of ETR7 that I use for several caliber

John, Where do you get your screens?

Thanks to all for the feedback. I seem to have gotten side tracked lately working on loads for my 45-70. So many loads, so little time! When I get back to the 38-55 I'll let you know how it goes, for better or worse.

JS