When there are two identical entries in a .po file, how does the translation mechanism determine which one to use?

I was trying to translate some plugin when I see in their .po file there are two "Sign in"s. I believe WordPress uses __ to parse text that needs to be translated.

So when codes like

__('Sign in', 'buddyboss-theme')

is executed, how does it know which "Sign in" entry in the .po file is the one to look for?