I've taken a closer look at this and found that if I do a manual update as described then I also get two installation dialogues.
In my case one is directly covering the other and so I only see the one unless I drag it from it's default location to uncover the other.
It doesn't matter how many times I try it the one is always directly covering the other for me, so wouldn't be noticed unless you drag the top one aside.
From what @Sarexsays about sometimes getting an error writing the same file then it appears that it is actually doing two installs at the same time for some reason.
Whilst that wouldn't generally be a problem (apart from the occasional write-the-same-file error) it shouldn't be happening.
i have ccleaner set to check for updates but not to install the update automatically, so, when there is a prompt to update ccleaner, i then allow the updater to install the update. (i have firefox set the same way, to show when an update is available but to allow me to select to allow the updater to install the update)
when there was an update for updating to build 5.89, and 5.90, i would see an error-message when the update was being installed, but the program seemed to be installed properly none the less. i didn't see the same error-message when installing the build 5.91 update.
i think that was the problem, what nukecad said, that there were two instances of the update running, but i didn't notice that since the GUI's were on top of each other.
the error-message that i would see was "couldn't initialize plugins," something like that.
this issue needs to be brought to the developers' attention, so that it can be fixed to where, when updating ccleaner, two instances of the installation do not run, together, at the same time.
As said above it has already been brought to their attention.
How high a priority it is will be a question for their manager to decide.
But as it's only ocassionally causing that 'error writing file', which is easily cured by trying again, then it"s more of an annoyance than a big problem and I don't suppose it will be high up the job list.
if the developers were aware of the problem they would have fixed it.
obviously the developers have erroneously been led to believe that there is no problem, despite the fact that people keep reporting it over and over and over and over, month after month after month after month, endlessly. give it another 7 years. eventually, someone at avast will get a clue.