The better your Current Season parts are, the less your Next Season's parts will improve.
Cloning parts instantly improves your Current Season parts, but as a resut, your Next Season progress will be slower.
You also want to be careful not to get your next season design too high. If you do, when the next season arrives, your development rate for the following season's design will be severely hamstrung. I started a season with all parts at 100%, and put 100% of my design emphasis on the next season, I was only getting 1% gain per update. The developers do this intentionally, so that you can't have 100% cars every season.
