Curran's last appearance for England in any format came in November last year and the allrounder has not yet featured under head coach Brendon McCullum.
Opening batter Duckett has been given and additional week to rest ahead of tour to Australia and New Zealand after playing all three formats England in the home summer.
In one final change to their squads, England have withdrawn seamer Matthew Potts from the T20I series in Ireland so he can play in the County Championship with Durham ahead of the Ashes.
Jordan Cox was added to the squad to face Ireland earlier this week but is not due to be involved in the series against South Africa.
England have already lost the ODI series against South Africa with one match left to play in Southampton on Sunday with the T20Is set to begin in Cardiff on Wednesday.
Updated England T20I squads:
vs South Africa: Harry Brook (captain), Rehan Ahmed, Jofra Archer, Tom Banton, Jacob Bethell, Jos Buttler, Brydon Carse, Sam Curran, Liam Dawson, Will Jacks, Saqib Mahmood, Jamie Overton, Adil Rashid, Phil Salt, Jamie Smith, Luke Wood.
vs Ireland: Jordan Cox, Sonny Baker and Tom Hartley replace Harry Brook, Jofra Archer, Brydon Carse and Jamie Smith; Bethell replaces Brook as captain.
England captain Harry Brook says his dropped off catch off Sanju Samson went a long way to his team losing their T20 World Cup semifinal against co-hosts India in Mumbai.
Sam Curran feels England need to be on top of their game if they are to beat India and silence the home crowd in Thursday's T20 World Cup semifinal in Mumbai.
Will Jacks says England are relishing life under Harry Brook's captaincy after booking their place in the semi-finals of the 2026 T20 World Cup with a perfect Super Eights record.
England completed an unbeaten Super 8s phase at the T20 World Cup after a thrilling four-wicket victory in the final over against New Zealand in Colombo on Friday.
