Duncan was set to play in an U19 friendly on Sunday but was called up to the under-20 squad after centre Jonny Kennedy (Sale Sharks) was ruled out with an ankle injury.
The injury means that outside-centre James Johnstone (Currie), who started against Wales, takes Kennedy's place at outside-centre, with Duncan taking the now vacant place on the bench.
Scotland U20 head coach Eamon John, said: "James [Johnstone] brings a different dimension to our attack. He has good feet and good hands which he showed against Wales to create Tom Brown's first of two tries that night."
On the prospect of fielding Glasgow Warriors latest addition, John added: "Duncan is a bright young prospect, he has settled into the camp very well this week and, having signed for Glasgow Warriors yesterday, he's now presented with another great opportunity."
National Academy Manager and Scotland 7s head coach, Stephen Gemmell, added: "This call-up follows a really big couple of weeks for Duncan and is good evidence of a successful partnership with Scottish Rugby, the west of Scotland Area Institute of Sport and Duncan's club [Glasgow Hawks] and shows that in Scottish rugby, if you're good enough, there are vehicles for you to accelerate your development."
Scotland
15 Tom Brown (Edinburgh Accies)*
14 Oliver Grove (Worcester Warriors)
13 James Johnstone (Currie)*
12 Alex Dunbar (Selkirk)*
11 Dougie Fife (Currie)**
10 Alex Blair (Edinburgh Accies)*
9 Alex Black (Leeds Carnegie)
1 Nicky Little (Hawick)*
2 Alun Walker (Currie)
3 Colin Phillips (Stewart's-Melville)**
4 Matthew Reid (Heriot's)
5 Robert Harley (West Of Scotland)*
6 Michael Maltman (Heriot's)**
8 Stuart McInally CAPTAIN (Watsonians)*
7 David Denton (Edinburgh Accies)*
Replacements
16 Lindsey Gibson (Melrose)
17 George Hunter (Glasgow Hawks)
18 Aaron Hall (Newcastle Falcons)
19 Callum Stidston-Nott (London Scottish)
20 Kris Hamilton (Caithness)**
21 Matthew Scott (Currie)**
22 Duncan Weir (Glasgow Hawks)**
* Denotes a member of the Scottish National Contracted Academy
** Denotes a member of the Scottish Rugby National Academy supported by the Scottish Institute of Sport