Removing and adding a variable gives an error hard to understand

This is the case that generates the error:

1. add a variable named VAR1, save
2. delete the variable, don't save
3. add the variable again
4. save

This happens because the saved list has a variable VAR1 that is also a "new" one.

We should at least give the name of the variable in the error message, otherwise the user must remove all the variables until the right one is found. A better solution is to automatically handle this case and update the variable instead of failing.