It is unclear whether Cunliffe or a staff member was responsible for his post.
Both candidates' announcements speak of pride in the Labour Party's history, values and people.
"I am excited by the opportunity this leadership contest provides to meet and hear from members and supporters and to put my case. I give my commitment that I will undertake this contest with respect for other nominees and to our Party," the posts say.
The only difference between the two posts is that Robertson's ends with a sentence omitted from Cunliffe's.
It reads: "Lets get into it."
A spokesman for Mr Cunliffe's campaign organisation said the double-up was due to a technical glitch.
"It's been taken down and he has apologised to Grant Robertson."
- nzherald.co.nz