“万万” reflects a preference in Chinese for using large, almost hyperbolic numbers and repetition to create emphasis. This is different from English, where emphasis is often achieved through tone of voice or adverbs like “really” or “very.” Using “万万” lends a statement a sense of formality, finality, and high stakes. A good comparison is the difference between a parent saying “Don't touch the stove” and “Under no circumstances are you to touch that stove.” The first is a simple command; the second is a grave warning. “万万” carries the weight of the latter. It's often used when principles, safety, or important rules are involved. This is why you frequently see it in official notices, hear it in serious advice, or find it in literature where a character is making a solemn vow or giving a critical warning.
“万万” is still common in modern Mandarin, though it carries a slightly more formal or literary tone than its counterparts like “千万” (qiānwàn) or “绝对” (juéduì).
Its connotation is almost always negative and serious. Using it in a lighthearted or positive context is incorrect.