lordnacho, as tptacek wrote below (which also applies to DSA);

>a full repeat instantaneously destroys security with a single pair of signatures

Roughly--assuming ECDSA parameters (H,K,E,q,G)--where H is a hash function, E the Elliptic Curve over finite field K w/ point G of prime order q. Suppose two different messages m and m' have been signed with private key x using the same (non-ephemeral) random nonce value of k.

According to ECDSA Signing these messages m, m' become signatures (r,s), and (r',s') where;

  r = r' = kG,

  s = (H(m) + x*r)/k   mod q,

  s' = (H(m') + x*r)/k  mod q.

Observe that,

  (H(m) + x*r)/s = k = (H(m') + x*r)/s'  mod q.

Or,

  x*r(s' - s) = s*H(m') - s'*H(m)  mod q.

Which allows us to recover the private key x.

Since,

  x = s*H(m') - s'*H(m) / r*(s' - s)  mod q.