Custom prompt prints twice in PowerShell Core 6.0 on Ubuntu

That's a bug in the version of PSReadLine that ships with Core. Here's the issue

You can install the prerelease version of 2.0.

Install-Module PSReadLine -RequiredVersion 2.0.0-beta1 -AllowPrerelease -Force

If you get a error about -AllowPrerelease you need to update PowerShellGet as well.

Install-Module PowerShellGet -RequiredVersion 1.6.0 -Force

CodeHunter