As I understand, ECB should not be used as encryption mode unless you are encrypting single blocks of data which are always unique and only are encrypted once.

I have a collection of ids represented by integers that I encrypt and transmit (individually) with the same public key using RSA/ECB/PKCS1PADDING. Sometimes the same ids are encrypted and transmitted multiple times but as far as I can tell the ciphertext is never the same.

Can anyone tell me how this is possible? As far as I understand, this would not be an acceptable scenario to use ECB, but why are the encrypted messages never the same even though the key and plaintext messages are?