Is there any protocol regarding votes on duplicate links?

I guess I'm not the only one that happens to this, I follow communities with the same theme in different instances, it's not unusual then to see the same link sent by the same person in both, not a crosspost, the same link sent separately. ¿Is there any, let's say, correct protocol on how to interact with this? You know, do I vote for both? One positive and the other I ignore? What if I want to comment? Do I make the same comment on both?