Can a MITM during Diffie-Hellman key exchange manipulate both sides to generate symmetric secrets?

Is it possible for an attacker on a Diffie-Hellman key exchange to manipulate both sides in a way so that the secret generated on each side is identical?

Or put differently, would it be possible to detect an attack via MITM if we can detect via a different channel that the secrets of both parties do not match?

Can a MITM during Diffie-Hellman key exchange manipulate both sides to generate symmetric secrets?

Let's assume that the MITM attacker can create an exchanged key that is the same on both sides. I.e., the party $A$ gets $g^t$ from the attacker and calculates $g^{at}$ and party $B$ gets $g^u$ from the attacker and calculates $g^{bu}$ such that $g^{at} = g^{bu}$.

If the attacker sends the identity element, then both sides will have the key as the identity element. This is a trivial solution and detectable (Other than the trial solution needs a reduction...)

would it be possible to detect an attack via MITM if we can detect via a different channel that the secrets on both parties do not match?

On the Major systems, protection from MITM attacks is done with certificates. Sometimes we do TOFU ( Trust On the First Usage) like in Signal ( and in the weaker WhatsApp) and later validate the public keys ^*.

One simple solution is reading the hex values on the phone since it is harder ( not impossible ) to fake real-time speech.

_{^* Have you ever validated one key in the Signal ( or WhatsApp)?}