Tags. Use the tags. When you create a new meme, you need to give it an original title and tags that are descriptive of the meme. This meme is a good example of how NOT to submit a meme. Tags are generated before you even submit it, as is the title.
When you make a meme, don't submit it immediately. Instead of hitting the submit button, hit the Go to page button. (of course, after you proofread. at this point, you can hit the edit button and correct any spelling mistakes) Once your meme is complete, go to the image page, select edit image, add a descriptive, and, if possible, witty title. Add your tags. for this I'd recommend tags to include meme ideas and reposts. Update that info, and click on the meme idea tag first.It will list the highest rated memes from that day backwards. Select latest, and view what's there. If you see a meme similar or the same as yours, open it and see when it was made. If Raydog made it yesterday, it would be a repost, and shouldn't be posted. IF, on the other hand, you find the exact same meme, but it was made a year ago, it shouldn't be considered a repost. I would edit the title to include that it was posted by another user a year ago (you can edit the title after updating)
Hope this helps.
BTW, if someone says it's a repost, and even provides a URL of it, check the tags. Chances are, they OP didn't post it with tags, or they used a different tag. But you have done your due diligence. If the commenter still rags on you, (and there's one or two that will) you can ignore them, or, if they are especially abusive, downvote and flag their comment.