I had this happen once on CO in 2007 or so. Got VDB from a flight after boarding, and took the next one out. I would up getting credit for the original and the new one. I assume this was just a random, lucky mistake. Plus I suspect the technology/tracking is better now.

I think your answer is it could happen by mistake in very rare cases. You might be able to call into the mileage desk and convince them of an error as well once in a blue moon, but I doubt this is a long term plan.