The phrase "have previously discussed" is correct and usable in written English.
You can use it to refer to a conversation or topic that you and another person have already talked about. For example, "As we have previously discussed, the deadline for this project is next week."
Since we have previously discussed the problems with stuff-thing or plural-thing dualisms, we will skip discussion of the difficulties of this dualism as such.
We have previously discussed these documents.
The two have previously discussed how they decided to continue with the campaign soon after they received the diagnosis.
We have previously discussed Voyager 1 data during the solar inactive period 1995 to 1998.
Mark and I have previously discussed the disproportionate number of comedians who are adopted – Stewart Lee and Rhona Cameron among them.
Given these issues, as I have previously discussed, I still believe that the F.D.I.C. call numbers are a better barometer of small-business lending.
I have previously discussed here what a shame it would be if we could no longer snoop around people's bookshelves (snooping on people's hard drives seems an entirely different order of snooping).
Speaking of the common ground over extending middle class tax cuts, Obama says if the House Republicans want to offer a bill along the lines they have previously discussed, he's ready to make it law: Let's not wait.
This I have previously discussed, including in a review I wrote for the professionally reputable medical journal The New England Journal of Medicine (August 18 , 1994 of the volume entitled The Oldest Old (Oxford University Press, 1993).
We have previously discussed VKT.
As I have previously discussed, VCs follow the innovation.
