Hello Mike
ok,Mike ,this is a difficult question to answer.hehe,but i 'll do my best to answer it.
there are two situations。
The first one :Mostly ,there are only girls say "辣辣的”。the duplication of the adj.contain a meaning just like some girls are coquetrying.I think it's a little strang for boys to say some duplication of adj. words.
ok,there are some other words like this,“美美”
the second one,We can say the duplicatian of adj. is more emotional, but the 很 is to express a strong feeling.look thess two sentences below:
我慢慢的走。Vs 我走的很慢。
o(n_n)o...Sorry ,it's too difficult to express it in English.haha .