Translation has duplicate admin content listings
Yes, that's expected - until filters are applied, that page is designed to list all content on the site, and separate translations are considered separate content.
Assuming you had 10 translations for each piece of content, yes, you'd see 10 times the number of listings as you would if you didn't have the translations.
That page is powered by a View. You can edit that View and do what you want with it - there's a language filter, maybe you want to set the default for that to the default language, thus allowing people to still browse the other translations using the filters if they want, but keeping things to the default language for most cases. Maybe you want to leave it as-is, but add a "Language" column, to make it easier for admin users to distinguish. There are a lot of options so you can probably get it configured in a way that makes sense for you.
No, there's no other way to translate content.