Shorter answer (less reading)
Highlight that data you want to work with, and click the Remove Duplicates option on the Data tab of the ribbon (07 and up). Then select the column(s) you want to search for duplicates.
...I just discovered this option a while back ... Right after spending an entire day manually sorting down a 7,000 item inventory dump.
Well this prob and many not dis-similar ones can be tackled with "intermediate skillz"! "Not all these steps all apply, but in holistic theme they do".
1. So the offending Duplicate Column is Column H. So during all this, save version controlled copies so if you mess it up learning you can go back! Copy 1a sort by col H.
2. Create a new column, whether it is Col I or Col AA or whatever, and do something like "if (Cell to my left) = (cell to my left and up 1), then "dup", else "clear".
-- You can't quote literary and quote an excel command but that's close enough to give the gist.
3. So then hopefully the first of anything always goes through, and all the dups get tagged as Dup. So then you save 1a at that point. Then in a fresh new copy 1b or 2a or whatever, you SORT BY DUP. So ALL the dupes float to the top!
4. Delete the dupes. Even on my aging comp or the one at old work, fifteen seconds because after about ten scrolls it starts flying by! Save that copy.
5. Create a fresh new copy. 3a, or whatever. But Bump the Version Number. Because by now that should have totally nuked all the dupes from one column. You can repeat as needed for other columns. But that's the method.
6. Save a final "Production Copy" after you've fixed the usual junk that just happened to catch your eye, irrelevant to this process.
Hope that helps!
I'm not a total "expert", I don't script Excel much, but I came to believe in it after my last job and now I am suspicious of anything "manual"!
Bonus - Rules of thumb
A. The first three letters of RIS(C) (No Chip). Reduced Instruction Set Winz! Fatality! (Mortak Kombat I miss you 1995!) So even if you can't totally kill the problem, there's a darn good chance you can take the manual part down from say 23 actions X 1000 lines down to say 7 actions X 1000 lines.
B. An unbelievable incredible number of things can be done with Chained Logic. So (If you've earned the right!) tell your boss you need two hours to think, because if the manual version of something takes 4-7 days but a five column logic chain can do it in twelve hours, think about it! The secret is heuristic rules. "Precisely why am I doing this?" If it's just a total crap data set you're a bit stuck, but if the action is suspiciously familiar, then do junk like "Length-mid-characters 5-8;;if they are such and such, flag to just kill".
(Totally not valid Excel Syntax, but 15 min will give you the right one.) Point is, For ex, if your correct data HAS to be 1112223333 for say a phone number but it all comes in 111-222-3333, characters 4 and 8 "cannot be dashes", so set up your five-seven column logic chain to kill them.
Whew! Enough for now! Just be Werry Werry Leery of manual junk in Excel/clones!