Currently if you attempt to import a new feature class into a feature dataset within a geodatabase that has the same name as the feature class you are importing anywhere in that entire geodatabase you get a very generic error message (Error 000210 - Failed to Convert). It you attempt to use the Feature Class to Feature Class to bring it in, you get a different error (Error 000594 - falls outside of geomtry). Both of these point you in the wrong direction in attempting to find the true problem.
So the request here is to add a better and more meaningful error message, something like "target already exists". From what I have heard from others this is a common problem and many have spent many hours troubleshooting when a simply a better error message could have prevented this.