Elliptic Curve Diffie-Hellman (ECDH) with

Public parameters: `Ep (a,b)` and `G  = (x, y)`  
Private Keys: `Na`, `Nb`  
Public Key: `Pa = Na x G`, `Pb = Nb x G`  
Secret key: `k = Na x Pb = Nb x Pa`  

Is the algorithm still secure if public key Pb is used more than once with different private keys Pb? Or are there any requirements to use use a public key more than once in a secure way?