I have a seed grown T. rivale that has a darker center than the authentic 'Purple Heart', the latter of which I was generously given by a Seattle friend with a well-deserved reputation for reliable naming of his plants. The seedling is, in my opinion, considerably better — more "purple heartish" you might say — than the clone. Moral: sow lots of seed from many sources and you too may find an 'Improved Purple Heart' among them. (Or should that be 'Purpler Heart'?)
The original does multiply well vegetatively. I don't yet know if that's true of my seed-grown one.