Setting the default desktop mode to duplicate/mirror mode is not possible.
It depends on your graphics adapter, some graphics adapter has mirror mode by default and some has extension mode by default.
Unfortunately in the newest version, this is impossible because for Android client keyboard does not work properly.
TAB,BACKSPACE,SPACE,ENTER,ARROWS keys are not working.
Do you have both spacedesk Driver (v1.0.18) and Android Viewer app (v0.9.81) to latest version?
Are you using the software keyboard? Or hardware keyboard?
If it’s software keyboard, can you tell us the keyboard app type/name (e.g. is it a GBoard)?