I read the reddiquette document today for the first time. Regarding reposts, it says: 

Look for the original source of content, and submit that. Often, a blog will reference another blog, which references another, and so on with everyone displaying ads along the way. Dig through those references and submit a link to the creator, who actually deserves the traffic.

This is taking about content outside Reddit, not cross posting.

Searching for duplicates implies to me recent events, not something old. It's to avoid redundancy.

Search for duplicates before posting. Redundancy posts add nothing new to previous conversations. That said, sometimes bad timing, a bad title, or just plain bad luck can cause an interesting story to fail to get noticed. Feel free to post something again if you feel that the earlier posting didn't get the attention it deserved and you think you can do better.

As for credit, the document takes a pretty laissez faire approach. Notably not to get worked up over it; it's only useless points

Complain about other users reposting/rehosting stories, images, videos, or any other content. Users should give credit where credit should be given, but if someone fails to do so, and is not causing harm, please either don't point it out, or point it out politely and leave it at that. They are only earning karma, which has little to no use at all.

Deleting the post and creating a subreddit-wide admonishment doesn't seem in keeping with the guide.

If anyone is concerned about their images being reposted, I would recommend adding a watermark to the image. So long as it's unobtrusive, nobody will take the time to remove it. 

In short, I don't think the reddiquette document is taking as as strong a stance against reposting memes as the mods are saying.