I just upvoted a comment in a community hosted on my site, and in the nginx log I can see:

ip.address - - [19/Feb/2024:14:33:31 +0000] "POST /inbox HTTP/1.1" 200 0 "-" "Lemmy/0.19.3; +https://endlesstalk.org"
ip.address - - [19/Feb/2024:14:33:43 +0000] "POST /inbox HTTP/1.1" 200 0 "-" "Lemmy/0.19.3; +https://endlesstalk.org"

Dumping out the activity, I can see:

... (single vote from account on another instance) ...                   
{
  "@context": [
    "https://join-lemmy.org/context.json",
    "https://www.w3.org/ns/activitystreams"
  ],
  "id": "https://endlesstalk.org/activities/like/f102d062-dfe9-4ece-96e8-310a09b249ba",
  "actor": "https://endlesstalk.org/u/freamon",
  "object": "https://sh.itjust.works/comment/9061222",
  "type": "Like",
  "audience": "https://lemmon.website/c/tails"
}
... (single vote from account on another instance) ...        
{
  "@context": [
    "https://join-lemmy.org/context.json",
    "https://www.w3.org/ns/activitystreams"
  ],
  "id": "https://endlesstalk.org/activities/like/f102d062-dfe9-4ece-96e8-310a09b249ba",
  "actor": "https://endlesstalk.org/u/freamon",
  "object": "https://sh.itjust.works/comment/9061222",
  "type": "Like",
  "audience": "https://lemmon.website/c/tails"
}
... (single vote from account on another instance) ...        

It’s the same for comments too (if I grep the dump for other people’s comments, there’s only one, but there’s two for comments made by me)

EDIT: can also see dupes from lemmy.ca (they actually sent the same Like 3 times).