You can exclude things rather easily by following the steps in the User's Guide (Help > User's Guide), Alex. Whether I'd do that is another thing.
In general, I really feel you're better off with duplication, preferably to different physical drives. The idea isn't to 'save space' (which is relatively inexpensive), but rather to save your data - and if you have a failure on one backup it's much less likely the other one will be bad.
If you value your time - and I know I value mine - it's well worth a small investment in drives to ensure that you'll be able to recover as quickly and completely as possible, even in the event that something goes wrong.