Theory:

I have two lists with equal number of items:

```
L1 = [0,1,2,3,4,5,6,7,7,8,8,8,9,10,10]
L2 = [12,412,51,523,52,54,65,74,35,22,14,1,3,76,159]
```

How can I find the duplicate items inside `L1`

get their index, and use that index to aquire the corresponding item in L2. Then get the value of these items from L2 and sum them up.

After that remove the duplicates from L1 or create another list (say L1a) without duplicates. Also create another list (L2a) where the items with indices equal to the duplicates from L1 are replaced with their sum.

Practical case:

I have a number of lines, such that all lines are in XY plane and parallel to Y axis.

Ergo, they all have constant X coordinate, but some lines just like the values in L1 overlap but have different lengths.

What I need is a sum of the lengths for each unique X coordinate.

Thanks a lot to all who reply in advance.