Pick the index in list, but there are same items

Hello, everyone. 

I have a question about that how to pick the index out if this item's value is same as it's previous item value.

Using the  "index in list  thing"

                "                     list"

There is the sample:

Index:      :  1  2  3  4  5

Set a list : { 1, 2 ,3, 2, 1}

How can I pick the index (4 and 5) out by using "index in list  thing"        ?

                                                                      "                     list" 

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the index in list block provides the index of the first item found, if you have duplicates, this block will not help. But why do you need that index?

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and you always can write your own custom procedure...

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Thanks for your reply @Taifun . Because I am doing a small project. In my project, the order of the item is quite important, I need to change the items (value) in the list one by one via different calculations. So if I can't get the index except first index, that means I can't change the following items either. I use the index to decide which item i should change.

I am organizing the concept of my all project, then I will post later, I believe that will be easier to understand my question.

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refer to: http://ai2inventor.blogspot.kr/2017/03/modify-properties-of-certain-index-in.html