Say you are running a profitable campaign on a site and want to run on the other placements on the same site (for example both NTVA and NTVB, or both mobile footer+header etc) to up the volume.
For simplicity sake using the NTVA + NTVB example, are you using different landing pages for each spot?
It seems like it would be redundant to use the same lander for both spots, but at the same time it's unlikely that a single user would click both your banners to see the same landing page?
If you think that offer will last long then there is nothing wrong in testing. Otherwise it just doesn't worth the time to create other LPs, test and optimize 'em, if the offer is not going to make enough profits.
LeadCloak
Based on my experience, it's definitely better to use different LPs ... makes the campaigns last longer and more often than not, the ROI is also better ... but the second LP must be a good one too of course 
And make sure you're not using the same banner, when targeting multiple spots on the same site, it's the best is to rotate a few banners.