研究研究 / 高兴高兴【yánjiū yánjiū / gāoxìng gāoxìng】

A two-syllable verb repeated whole – AB becomes ABAB – means doing it briefly or casually: "a bit", "have a go at". It softens a suggestion or a request the way 一下 does.

The verb has to be something you choose to do. 知道, 是, 喜欢 and 认为 don't double.

ABAB, not AABB

A verb doubles as a block. AABB is the pattern for adjectives that describe (高高兴兴, 干干净净), and it means something else.

• × 研研究究 → 研究研究
• × 休休息息 → 休息休息

With 了

For something already done, 了 goes between the two halves.

• × 他考虑考虑了 → 他考虑了考虑

一 can go between a one-syllable verb and its double (看一看), but not between two-syllable ones.

• × 研究一研究 → 研究研究

Adjectives as ABAB

A few adjectives for pleasant states double as ABAB: 高兴, 暖和, 凉快, 热闹, 轻松. They then act like verbs: "let's have a bit of fun", "warm up a bit".

AABB describes; ABAB does. 他高高兴兴地走了 is "he left happily"; 让他高兴高兴 is "let him have some fun".