I had my thought in the past. "I thought" is past, versus "I think," which is present.
And in that time in the past, when I had that thought, I though something had already happened even FURTHER in the past, so it was "had seen" versus "have seen."
If I'm looking at something with you right now, I could say "I think you have (already) seen this, right?" but now we're talking abou