A picture says a thousand words:
Pictured is the result of using the volume shortcuts on my laptop's keyboard several times to reduce the volume. This doesn't always happen. Sometimes it will spawn a few windows and then correctly modify the last one, i.e. resulting in only one popup displaying the volume as I change it.
I'm running Xubuntu 16.04 with kernel
4.10.0-32-generic on a Lenovo ThinkPad X240.
My mild guess is that it may be tied to the KXStudio repositories, which I use, but I don't think I've installed anything other than lmms and its dependencies from it. I don't recall the issue from before these but I'm not quite sure as to how it would affect, given that I only use the repositories for lmms.