When exporting po file templates, with the default option (Source text only, no translation) I've found these issues:
1. We get duplicated strings.
To get this, you just need to add two languages and add a translation of the same string for each.
Looking into the query, it seems either there's one DISTINCT missing or one join not needed (we wouln't need to join locales_target here).
2. Then, reloading the page, we get multiple warnings like these:
Warning: addcslashes() expects parameter 1 to be string, array given in Drupal\Component\Gettext\PoItem->formatString() (line 269 of core/lib/Drupal/Component/Gettext/PoItem.php).
I suspect this last one is related to the singular / plural handling in PoItem class.