>"I've sent a paper to Nature, which got held up because our reviewer was about to send the same stuff to Cell."

I suspect this reflects a much deeper anti-scientific attitude that I have seen in academia. If it was the exact same methods it would be great to see independent groups publish their results. Using different approaches to study the same thing is also great.

The only way I could see a problem is if they literally copied your results without running their own experiments/analyses.