Repeatedly citing same item of personal communication

Question

The APA style indicates that personal communication should be cited in the format "J. Doe (personal communication, M dd, yyyy)" in-text, without a bibliographic entry. I was wondering what the best solution to citing such an item repeatedly is?

Example:

J. Doe (personal communication, August 21, 2017) proposes that concept X is inherently meaningless without mechanism Y.

[...]

Doe (2012, 2015, 2016) presents mechanism Y in terms a, b, c. We could therefore assume that this predicts concept X to exist independently of Y, nothwithstanding J. Doe's (personal communication, August 21, 2017) objection that doing so would lead to undesired behaviour Z---it is what strictly follows from the system proposed in Doe (2012, 2015, 2016).

Is there a better way to refer to the same personal communication the second time? Options like J. Doe (op. cit.), J. Doe (ibid.) etc. seem to not really work semantically (given what kind of object op. cit. and ibid. refer to) and be potentially ambiguous with Doe (2012, 2015, 2016), although I suppose the initial of the first name here may disambiguate.

Are there any established ways of dealing with this elegantly?

[Edit:] I should add that I have to refer to a different instance of personal communication with Doe elsewhere, so simply citing "Doe (p.c.)" later on would be potentially ambiguous, unless it somehow only referred to the most recently cited instance of personal communication.

[Edit 2:] To provide some context, we're talking here about a situation where the papers by Doe (2012, 2015, 2016) outline a particular theory but leave some aspect of it poorly defined or ambiguous. The private communication entails asking Doe for clarification as to how he intended that ambiguous aspect of the theory to be understood/implemented. So this is not really about relying on data or sources that are unavailable, it is about giving fair shrift to Doe's theory, and making clear as a starting point both for my own work and others how the original point was intended. If this is not taken into account there's a real danger of just attacking a self-made straw man here, and neither Doe nor I nor others in the field would probably be happy with that. It seems inevitable to me that full disclosure here entails citing the personal communication on more than one occasion, namely whenever Doe's disambiguating communication comes into play.

Answer 1

Without knowing the context, what you are trying to do seems poor practice to me. "This is important because person X said so in private" is impossible for peer reviewers to verify, and hence not authorative when written in a paper.

Answer 2

If you are essentially just mentioning privately stated opinions, then don't. Constantly citing the private opinions of others is bad form and constitutes you ducking the proper work of performing a literature review to establish the importance and shortcomings of your work in a proper, verifiable, and objective fashion. You are tacitly telling the reader "I don't really know what I'm doing but I think this guy does." Avoid this. Your paper is your work, your voice, your opinions. Do the literature review and make your own case about your own work.

If this private communication is significant to your work in a substantive way (proofs of theoretical results, experimental data, etc.), your options include:

• Ask them to be a coauthor if their work is essential to a significant fraction of the work.
• Ask them to write an appendix to attach to your work that contains the relevant material.
• Ask them if these are things that exist in a preprint, article under review, or in fact published article. If the material is reasonably accessible to the public, then a reviewer will be able to check it and verify that it provides what you claim, and if it is trustworthy enough to stand on its own. If it is not, you should ask if they would approve of you providing a (confidential) copy to the reviewers of your own work.
• Include the result, with full details of the proof, in your own work, saying something like "The proof of the following was kindly provided to us by X in a private communication." It is not uncommon for an author to include such a line, or any variation thereof, such as "The author thanks X for their helpful discussions", which is a polite recognition that they were important to the development of your work (and possibly why you even started it), but not to such an extent as to warrant coauthorship. Most researchers don't come up with things in a vaccuum, and are guided towards problems by their conversations with others—that's part of why we talk to each other about our research topics, even the incomplete ones. And sometimes we need one important lemma to get anywhere, and someone else we talk to figures out a proof but doesn't otherwise contribute to the work. I've also seen things like "We learned several of these results from X, but since they do not seem to appear in the literature we state and prove them in full." This sort of statement can be very common for "folklore" results: things the biggest experts all seem to hold as common knowledge, but yet no one has ever actually published.
• Failing all of those, you're in a bad position. It's very undesirable for you to make claims that are impossible to verify, and going ahead and reproducing things this other person has done when they didn't want you to, while not necessarily "wrong", may strain your professional relationship with that person. I would expect this scenario to be very unlikely, but there's always tales of that one intransigent curmudgeon... While "private communications", especially in the pre-internet age, are hardly a new phenomenon, generally these communications have been provided to reviewers whenever they contain material that needs verification (a proof of something, in particular), and as such this is typically seen as a natural and non-threatening matter.