Jared
Thank you for the assist - I was able to get to the bottom of this, kind of. I did indeed leave out important context in my original post -...
Actually, I wonder if this is related to downloadButton being in sidebarPanel, which should contain input controls only?
Thanks - there are no warnings about duplicates (I tried refresh also).  The above was a pretty compact breaking example. If you remove the sidebarLayout/sidebarPanel/mainPanel the button won't be...
See here for example https://cran.r-project.org/web//packages//quarto/vignettes/hello.html
That said, I found this quote from Packages 2e: > In vignettes, more than anywhere else, the answer to “But how do I do X?” is often “Don’t do X”.