ElGamal scheme signature: if private key a mod p is equal to private sig. key k mod p-1, can an attacker notice and determine the value of a?
No, it doesn't provide extra information on the private key.

Note that there is no restriction on the private key value, you just have to ensure that $gcd(k, p-1) = 1$ and that $k$ is only used ONCE. Otherwise the private key can be easily found: see this link which explains how it fails for DSA (this is the same reasoning for the ElGamal scheme).