Explanation
Indicates 'as good as', 'virtually', or 'practically' the same as something.
Formation
Noun + (も)同然だ, Verb-casual + (も)同然だ, etc.
Detailed explanation
「～も同然だ」 means something is practically the same as (X). It emphasizes that although it may not be literally identical, there is hardly any difference in reality.
Examples
Kare to wa nagai tsukiai dakara, kazoku mo douzen da.
I've known him for so long that he's practically family.
Mainichi chikoku suru nante, kite inai mo douzen da.
Being late every single day is as good as not coming at all.
Kanojo to watashi wa osananajimi de, shimai mo douzen no naka desu.
She and I are childhood friends, practically like sisters.
Konna ni sukunai kyuuryou ja, tadabataraki mo douzen da.
With such a small salary, it's practically like working for free.
Practice
Grammar content sourced from hanabira.org.