Though I agree that 2046 is a rehash of the ideas in In the Mood for Love, I see that as precisely the point. The two films both exist in two adjacent universes, with one creating the other.

In the Mood for Love is the dream past of Lueng's character in 2046 and 2046 is the dream future of his character in In the Mood for Love. 2046 seems to deal with exactly this when Lueng dreams/writes about "2046" as a place. This "2046" is another version of the same love/loneliness story that seems to infect everything he does.