@Carolgreen986,

The meaning is the same, only the degree is different. Doubling the characters raises the degree.

It's like saying 很平淡 (very dull) or 彻底明白 (totally clear) or 相当开心 (extremely happy).

It's like having an adverb of degree built right in.

BTW: these are all adjectives, so you can't say 我明明白白你的意思。

You would use this phrase to say something like, 我懂你的意思因为你已经把你的意思说得明明白白.

Hope that clears things us. 希望你对以上的这些词组的含义清清楚楚的.

(All non-native speaker disclaimers apply)