How much duplication does it realistically need? The line is very short (9 minutes from Eagle Juction to Doomben). I reckon duplicating Hendra-Clayfield, and reinstating the second platform at Doomben, would be sufficient. Hopefully, it could be done without having to demolish or otherwise interfere with the Sandgate Rd overpass, or it becomes substantially more expensive.
But, for some reason, QR doesnt like going after the low-hanging fruit, as can be seen by their consistent failure to duplicate Sandgate-Shorncliffe.
Aren’t there three? Eagle Junction to Doomben, EJ to Airport, Northgate to Shorncliffe.