Is it Possible to Generate a Duplicate DH Session Key from 2 Pairs of DH Keys?
If you're doing Diffie–Hellman in a group $G$ written multiplicatively with identity element $e$, say $G = (\mathbb Z/p\mathbb Z)^\times$ for some standard large safe prime $p$ and $e = 1$, then Eve/Mallory can simply supply $e$ as their public key to both Alice and Bob.  Then the shared secret group element is $e^a = e = e^b$.

There is a way for Alice and Bob to thwart this, by simply checking whether the peer's public key is the identity element.  But it's usually pointless because Alice and Bob will normally authenticate the entire transcript of the session—this thwarts the attack anyway because they will authenticate $g^a$ and $g^b$ as their public keys, not $e$.