What is the purpose of verb reduplication?
The examples you gave are not verb reduplication
phrases like 笑笑，看看，走走 or 吃吃 are verb reduplication, it serves to indicate the verb is in a slight degree or a casual manner

I sneezed a big sneeze
She sleeps a long sleep

These are just a same word as verb, and then as noun (object) in a same sentence

the first sneeze is a verb; the second sneeze is a noun

the first sleep is a verb; the second sleep is a noun


了 is not a verb, it is a 'verb particle' that indicate 'completion of the action' or 'change of situation.
In your 西游记 examples

指 = point

指了= pointed (the pointing action is completed)

笑了笑 - the first 笑 is a verb; the second 笑 is a 'verb classifier' for counting how many times the subject smiled (since there is only one smile, the action is in a slight degree or casual manner)

晃了晃 - the first 晃 is a verb; the second 晃 is a 'verb classifier' for counting how many times 金箍棒 was swung (since there is only one swing, the action is in a slight degree or casual manner)

转了转 - the first 转 is a verb; the second 转 is a 'verb classifier' for counting how many times 眼珠子 was turned (since there is only one turn, the action is in a slight degree or casual manner)


Since the second 笑, 晃 and 转 are verb classifiers, they work with counting words: 笑了(一)笑, 晃了(三)晃, 转了(八)转

抢了我的东西 (seized my stuffs) - '抢了' is a completed action; 我的东西 is the object
抢我的东西 - (seize my stuffs) - '抢' is a verb; 我的东西 is the object (no indication of either the action is completed or not)

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'做了坏事' (have done bad thing) - '做了' is a completed action; '坏事' is the object
'做坏事' (do bad thing) - '做' is a verb ; '坏事' is the object (no indication of either the action is completed or not)

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'得到了满意的答案' (have got a satisfied answer) - 得到了 is a completed action; 满意的答案 is the object
'得到满意的答案' (get a satisfied answer) - 得到 is verb; 满意的答案 is the object (no indication of either the action is completed or not)