I need to learn what?

好好地. And not just 好. I've heard "tone morphing" with 宝宝, 姥姥, 奶奶, 爸爸, 妈妈。But the weird thing is that in each of those cases the tones all morph to third followed by first, irrespective of what the standard tone of each word is. Granted some of those are probably non-standard but I have heard them uttered by southerners and northerners alike.

E.g. 爸爸 would be ba3ba1.

Humm, good point, I think I've heard that too. It's a bit like rather than the reduplicated character becoming qingsheng, it becomes first tone.

Cantonese does this kind of "tone change" (not sandhi), though it merges it into 陰上 (high rising tone in Cantonese).

Matthews & Yip (1994)

In Cantonese, tone change...occurs due to a number of morphological and semantic factors... The main tonal change occurs in compounds and reduplicated expressions, in which a mid/high rising tone results. It occurs primarily where the second syllable of a compounds or a reduplicated word has a low-register tone.

Several examples follow, many along the same lines as the Mandarin ones above, as well as several other types: 試試 from 試一試, 啊- or 老- as prefixes for names, loanwords, and other curiosities.