$ rig add 3.5.3
[INFO] Running `sudo` for adding new R versions. This might need your password.
[sudo] password for xxxxxx: 
[INFO] Downloading https://cdn.rstudio.com/r/ubuntu-2004/pkgs/r-3.5.3_1_amd64.deb ->
    /tmp/rig/r-3.5.3_1_amd64.deb
[INFO] Running apt-get update
--nnn-- Start of apt-get output -------------------
Hit:1 https://nvidia.github.io/libnvidia-container/stable/ubuntu18.04/amd64  InRelease
Hit:2 https://dl.google.com/linux/chrome/deb stable InRelease                  
Hit:3 https://download.docker.com/linux/ubuntu focal InRelease                 
Hit:4 http://security.ubuntu.com/ubuntu focal-security InRelease             
Hit:5 http://cn.archive.ubuntu.com/ubuntu focal InRelease     
Hit:6 http://cn.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:7 http://cn.archive.ubuntu.com/ubuntu focal-backports InRelease
Reading package lists... Done
--uuu-- End of apt-get output ---------------------
[INFO] Running apt-get install
--nnn-- Start of apt-get output -------------------
Reading package lists... Done
Building dependency tree       
Reading state information... Done
gdebi-core is already the newest version (0.9.5.7+nmu3).
0 upgraded, 0 newly installed, 0 to remove and 10 not upgraded.
--uuu-- End of apt-get output ---------------------
[INFO] Running gdebi
--nnn-- Start of gdebi output ---------------------
Reading package lists... Done
Building dependency tree        
Reading state information... Done
Reading state information... Done
Selecting previously unselected package r-3.5.3.
(Reading database ... 311120 files and directories currently installed.)
Preparing to unpack /tmp/rig/r-3.5.3_1_amd64.deb ...
Unpacking r-3.5.3 (1) ...
Setting up r-3.5.3 (1) ...
--uuu-- End of gdebi output -----------------------
[ERROR] File exists (os error 17)

$ rig list
3.5.3
3.6.2
3.6.3
4.0.5
4.1.0
4.2.0

$ rig default 3.5.3
[INFO] Running `sudo` for setting the default R version. This might need your password.
[ERROR] File exists (os error 17)