"Rarer than rare" is a popular phrase used to describe something that is exceptionally scarce or unique. While the phrase itself is effective in conveying the idea of extreme rarity, there are many other synonyms that can be used to describe the same thing. Some examples include "scarcer than hen's teeth," "one in a million," "uncommon as a unicorn," "as rare as a blue moon," "hard to come by," "extraordinary," and "unprecedented." Each of these phrases highlights the same message but in slightly different ways, making them ideal for use in various contexts and situations where an emphasis on rarity is important.