I have seen this problem reported by others, including on this forum, but without answers which satisfactorily solve this problem, at least not for me (e.g. here and here). Also, I have had this problem on another laptop (an Asus) with Ubuntu 18.04, my current system is a Dell on Ubuntu 20.04. Aside from the applications I have installed, there is nothing fancy linux-wise on my laptop; as far as I can tell, it is pretty much "plain vanilla" (though I do have NVidia drivers that occasionally cause problems, my previous laptop did not have this and still had the problem).
The symptoms are as follows: a random key (i.e. this can happen on any key as far as I can tell: letters, numbers, arrows, delete, backspace...) acts as if it were stuck and the key "repeats" until another key is pressed. The repetition of a key that is actually pressed is a desired feature when voluntary, so solutions involving deactivating key repeats are not a proper solution for me (or others, I suspect). I could be wrong, but from what I am reading and noticing this does not seem to be linked to any keyboard defects but rather has the characteristics of a software issue.
This issue is similar to this one, except that it is not linked to the key being "fast-pressed" or the keyboard being "brushed". However, as indicated in the answer, and since the BIOS was involved, I have updated the BIOS, but the problem is still persistent. As far as I can tell, the system is all up to date (aside from the fact I am sticking with 20.04 for now).
Note that this happens randomly, often enough to be annoying, but not often enough that I cannot use the computer. I would say about once every 100 to 500 key presses...
Although I have seen comments (some of them a bit snide to be frank) related to potential problems with the actual keyboard and/or the user, I think this problem has been reported enough times by enough people with enough different systems / trying different keyboards to merit an answer which, hopefully, can provide enough insight for most users encountering this problem for them to be able to solve the issue. I am not saying it is easy to have a generic answer to this problem, but if by any chance someone has solved this and can indicate the steps for a diagnosis and a corresponding solution that would be greatly appreciated! Ideally, what I am looking for is 1. a list of common issues causing this problem, 2. a list of commands enabling us to know which of those cases corresponds to our situation, and 3. a potential resolution path or pointers. Of course, if this is not possible, then a specific answer for my situation would be nice...
[EDIT 1] in response to the two comments so far:
- I agree that it could be hardware in the sense "hardware dependent"; what I meant by I believe this is software related is that I do not think my hardware is physically or electronically faulty.
- When using LiveUbuntu via USB, the problem did not happen, whereas I had the keyboard issue in my first week of using the computer. This is not a 100% indication that this is not a hardware problem as the issue happens randomly from what I can tell. The problem has not been getting noticeably worse or better this last year of usage.
- That said, I have been unable to reproduce the problem with an external keyboard. It is tempting to deduce from this that it is a hardware issue, and once again if by the hardware we include drivers, etc. then I agree, but (again) I wouldn't conclude that something is physically wrong with the laptop's keyboard (this happened on another laptop of mine, and what I read from other users with this problem is very similar to my situation).
- I have dual OSes and have a partition with Windows, which I hardly ever use. But so far, I never had the issue on Windows.
- the fact that the issue was on two different Ubuntu OSes but also on two different computers does not scream hardware to me, but I am no expert in terms of OS, drivers, or hardware for that matter. Couldn't there be a persistent issue in Ubuntu? I have seen posts about this sort of thing dating back to 2012 or so...
- I realize the role we play in asking and answering questions on the stackoverflow family of websites, and I do play on the other side of the fence for the subjects I have some knowledge about. I am not taking answers for granted and am unsure what warranted that comment, but I will assume it is my phrasing and my use of the words "merit an answer". What I should have said perhaps is that this issue, I think, concerns enough of us that it should merit attention (rather than an answer)? What I am trying to convey is that this issue has been reported on different platforms and is dismissed as a non-issue. The merit of the question, IMHO, lies not in the fact that I am asking it but that several of us are. I hope this clarifies my intent. If this is still unacceptable then please close the question as meritless, I completely understand and hope to limit anyone's waste of time.
I have been using logkeys as a kelogger, and here is what I found:
- first, it seems I need many more keystrokes than I guesstimated before triggering these repeats. I believe somewhere in the thousands.
- second, here is what was in the log file after the phenomenon occurred:
I am not sure why the character mapping is incorrect, but when I typed the same characters and forced the last letter to repeat, here is what I got:
So what this shows is, first, that the keylogger does log those repetitions (which I guess won't surprise people in the know, but I wasn't sure it would), and second there seems to be a difference (aside from the number of repetitions), namely that the logger captured a [left] keystroke which did not happen just before capturing the repetition, however, the repeated key was the "c" in that sequence. (Note: 1. the actual characters typed were "nan", not "cȁc", and this is probably due to me using a Canadian keyboard mapping. 2. in the above, I have changed the "lower than" and "greater than" symbols by brackets  to represent those characters as these otherwise do not show up in this edit.)
[EDIT 3 ]
I caught another occurrence using the keylogger, and this is what was logged:
Where the last letter ("w", which mapps to "e", was repeated 7 times). There were no spaces or other characters typed after the last "w", at which point is stayed "stuck", so I am not sure why the extra characters (the space and then the 3 special keys) got inserted before the logger logs the repetition of the "w", but perhaps this can clue someone as to what is going on here...