I installed a more up to date mono from the package on their website and it seems like it works. The file ‘run-with-mono.sh’ in the mac Duplicati bundle suggests that only mono 3.0 is required but maybe that’s not true anymore?
$ /Library/Frameworks/Mono.framework/Versions/Current/Commands/mono /Applications/Duplicati.app/Contents/Resources/Duplicati.CommandLine.exe
See duplicati.commandline.exe help <topic> for more information.
General: example, changelog
Commands: backup, find, restore, delete, compact, test, compare, purge,
vacuum
Reparing: repair, affected, list-broken-files, purge-broken-files
Debugging: debug, logging, create-report, test-filters, system-info,
send-mail
Targets: aftp, amzcd, azure, b2, box, cloudfiles, dropbox, ftp, file,
googledrive, gcs, hubic, jottacloud, mega, onedrive, openstack, rclone, s3,
ssh, od4b, mssp, sia, tahoe, webdav
Modules: aes, gpg, zip, 7z, console-password-input, mssql-options,
hyperv-options, http-options, sendhttp, sendmail, runscript, sendxmpp,
check-mono-ssl
Formats: date, time, size, encryption, compression
Advanced: mail, advanced, returncodes, filter, default-filter-sets, <option>
http://www.duplicati.com/ Version: - 2.0.3.3_beta_2018-04-02
$ /Library/Frameworks/Mono.framework/Versions/Current/Commands/mono --version
Mono JIT compiler version 5.10.0.160 (2017-12/f3ac8fdeb37 Tue Mar 6 14:07:30 EST 2018)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: normal
SIGSEGV: altstack
Notification: kqueue
Architecture: amd64
Disabled: none
Misc: softdebug
Interpreter: yes
LLVM: yes(3.6.0svn-mono-master/8b1520c8aae)
GC: sgen (concurrent by default)