Help wiht INDEX function

Hi,

I have workbook which has three sheet.

sheet3 is getting updated beginning of the week with the data
sheet2 is getting populated by end of same week with the data

In sheet1 I am trying to compare the sheet3 and sheet2 for getting unique value.

In sheet1, with index & small function together I am able to get unique value from sheet3 (column B) but I am having problem with sheet2 data (column c) where I am not getting unique value

See attached file for sample

looking some help on this

thanks