Other than 秘密，are there two-syllable words in Chinese where each syllable is a different character with the same pronunciation?
I've come across words like this a couple of times.
逝世 shìshì
致志 zhìzhì
世事 shìshì
Alliteration (双声) seems to be quite common in Putonghua, and this is an extreme form of alliteration.
It's interesting how all these examples use the fourth tone.
There are more words like this but they tend to be less commonly used or literary words (书面语). Eg: 幽忧 yōuyōu
Edit: Mou某 gave two other good examples: 屈曲 qūqū; 实时 shíshí. And these aren't fourth tone, which is nice.
Edit 2: 全权 quánquán (from Becky 李蓓 's comment)
Edit 3: 意义 yìyì; 意译 yìyì; 异议 yìyì (from Olle Linge's comment)