As per discussion in another thread, I just realized that I don’t actually know which program to check to answer your question.
I have installed Duplicati2 for the first time in september 2017, and I checked the installation instructions only back then. Afterwards, I have not looked for updates. Now that I did, I learned that you require us to install mono-devel, not something called mono.
When I answered your question, I gave the version of mono, and ignored mono-devel. I have not installed or upgraded either of them, so I have for both the version that comes with Ubuntu 16.04. Apparently, the mono-devel version is recent enough for you (but mono is not).
This goes to show I had no clue of what mono is. Might I be the only one here?