A while back I built something like that for my old employer. It did most of the work on the server, but it did a good job of guessing column mappings, highlighted bad cells and let the user fix them, checked for dupes, and all the validations could be configured with a little xml, including multi-column constraints. I used Aspose for the excel import.

I've thought about making an updated version, maybe as a hosted service, if I can come up with something different enough so my old employer can't claim ownership. I did a little test marketing a while back and it didn't go that well, but maybe it just needs more experimentation.