I would write:
void loop() {
digitalWrite(led12, HIGH);
digitalWrite(led5, digitalRead(led12)); // write copy of 12 to 5
delay(1000);
digitalWrite(led12, LOW);
digitalWrite(led5, digitalRead(led12));
delay(1000);
}
I would write:
void loop() {
digitalWrite(led12, HIGH);
digitalWrite(led5, digitalRead(led12)); // write copy of 12 to 5
delay(1000);
digitalWrite(led12, LOW);
digitalWrite(led5, digitalRead(led12));
delay(1000);
}