The phrase "a topic once" is correct and usable in written English.
It can be used when referring to a subject that has been discussed or mentioned previously, typically in a context where the speaker is recalling or referencing that discussion.
Example: "We covered a topic once in our last meeting, and I think it deserves further exploration."
Alternatives: "a subject previously" or "a matter at one time".
The best example is the changing of mind on a topic once taken to epitomise foreign-policy fecklessness: development aid.
(Honor killings, a topic once again in the news in Britain, are the terrain of Webster's 1623 text).
Now, teachers who follow cognitive psychology are advocating a shift of priorities to spacing learning over time across the 3 years, rather than teaching a topic once and then intensely reviewing it weeks before the exam (Cox, 2016a; Wood, 2017).
A direct candid discussion about a given topic once a week that is not read off a teleprompter is priceless for the continual sense of a direct relationship.
The film's stars and creators have been making the media rounds and the buzz about the new film has inspired talk show hosts and news programmers to treat education like a sexy topic once again.
After you've given constructive criticism on a particular topic once or twice, you've probably said enough.
Long the butt of punchlines for his nasal-sounding voice and geeky demeanor, British Labour Party leader Ed Miliband has become a hot tabloid topic once again after giving a bizarre "robot-like" TV interview.
To achieve the objectives of the project, during a 2-year period from the beginning of 2012 to the end of 2013, we conducted 14 FGs on ICT-related products with community-dwelling older adults, with a frequency of two FGs on the same topic once a trimester.
For Ms. Lister-Jones, the prospect of exploring the topic once more, albeit as a monogamous couple, is a kind of double-edged sword: as unsettling as it is exciting.
Martin distinguished between the number of Twitter users who mention a topic just once or twice and those that tweet about it 20 or more times (the pink circles below).
In total, 14 FGs were conducted with community-dwelling older adults, with a frequency of two FGs on the same topic once per trimester.
