If someone says I love you and you reply “as do I “ it means I love you too right ? Or does it mean you love yourself ?
If interpreted literally, it would mean you loved yourself. Luckily, the meaning of sentences does not always depend on a literal interpretation of the words spoken.
This is called implicature in linguistics and refers to what is suggested in an utterance, even though neither expressed nor strictly implied (that is, entailed) by the utterance. This is part of the wider field of pragmatics.
"As do i" is more formal. It is correct but seemingly uncommon and the use of it should be done by not the locuter or our partner, e.g
A: i love you B: as do i
While in monologue, the form of as will not be inverted, like: A: she loves him, as i do
So, if you say "i love you" to your wife then your wife replies "as do i" or vice versa, it's actually ok as far as i'm concerned