Here is hopefully a little help. About all I have done is automate the "Remove Duplicates" step that you were doing manually. This is done via the following formula in Column-G:
This gets you to the highlighted duplicate values in column-H with no manual steps being needed. See rows 16 to 26 in the attached workbook.
I'm sure it's possible to do without the various "helper" columns and combine everything into one formula, but that will need more thought.
>>>>> EDIT:
I forgot the attachment. I also needed to change the A/B/C concatenation formula from =A16&B16&C16 to ="T"&A16&B16&C16. Without that the elimination of duplicates did not work correctly when the least significant digits of col-C were changed (Excel working in numbers rather than text I suspect).
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