I won't explain the whole procedure in detail because this is an AntConc discussion group (not an Excel discussion group!), but basically you need to use the Power Query Option:
1) Open Excel
2) Open Power Query (Data > Get Data > Launch Power Query Editor)
3) Choose your Excel table (New Source->File->Excel Workbook->Your file
4) In the new window, click Preview (to see the data) and OK to load the data
5) Select the column of data (the Kwic concordance results - merged into a single cell for each row)
6) In the ribbon, expand "Keep Rows" and select "Keep Duplicates" (this will remove the single entries)
7) In the ribbon, expand "Remove Rows" and select "Remove Duplicates" (this will remove one of each duplicate)
8) Click "Close and Load" in the ribbon to go back to the Excel worksheet
Once you know this, you can do it all in about 5 sec.
I hope this helps!
Laurence.
p.s. I really should make it an option in the AntConc advanced search settings. I would expect few people would know how to do the above unless they were a power user of Excel.