Trainer Cindy Alderson is banking on a weight drop to help Duplicity Jones turn the tables on nemesis Jileks Spur in Australia's longest flat race.

Duplicity Jones has finished second to Jileks Spur in two heats of the MackTow Marathon Series run at Sandown and Mornington.

On Saturday the pair clash again in the series final over 4000 metres at Caulfield.

The pair share favouritism at $2.50 with To Do ($7) the only other runner under double figures.

Duplicity Jones gave Jileks Spur 4.5kg over 3000m at Sandown on March 18 when beaten a half head and then gave him 2.5kg when beaten a half neck over 3400m at Mornington on March 28.

On Saturday there is only .5kg separating the pair but importantly Duplicity Jones drops from 59.5kg at Mornington to 58kg on Saturday.

While the weights favour Duplicity Jones, Alderson is concerned his racing style may leave him a sitting duck.

"If a horse like Jileks Spur has the stamina to stay with him and find a little kick they can generally get past him," Alderson said.

"No-one else has been to this distance before so it's unchartered territory.

"He and Jileks Spur appear to have the others that we've competed against covered at this distance range unless someone comes out of the woodwork."

Alderson said Duplicity Jones was likely to go for a break after Saturday.