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From [Selected-packages set (Portage) — Adding an atom without recompilation][1] (yes, embarrassingly the very link in the question, the word "atom" put me off...),

    emerge --noreplace xournalpp

```
--noreplace, -n
       Skips the packages specified on the command-line that have
       already been installed.  Without this option, any package atoms
       or package sets you specify on the command-line will cause
       Portage to remerge the package, even if it is already installed.
       Note that Portage will not remerge dependencies by default. This
       option can be used to update the world file without rebuilding
       the packages.
```

Used with `--ask`, the prompt will change accordingly to

    * app-text/xournalpp
    
    Would you like to add these packages to your world favorites? [Yes/No]

[1]: https://wiki.gentoo.org/wiki/Selected-packages_set_(Portage)#Adding_an_atom_without_recompilation