Last modified: 2014-05-23 01:39:41 UTC
This is a simple check that can help people avoid overwriting images with incorrect alternatives. This is sensible given that photographers often upload alternative versions of images, making confusion among the local files a significant possibility. Such mistakes will often not be obvious to peers.
There's often times where people upload a new version with different dimensions. I think there's enough false positives that we shouldn't give a warning here (But am open to having my mind changed, e.g. If a lot of people on commons wanted this I would change my mind)