Another option (which should result in less work each time) might be to try this.
I'm not an accountant, but I have assumed the taxid's will all be unique.
Assumes your data (on both sheets) is in columns A:D with the taxid being column D.
Assumes your sheets are actually named "main" and "sub"
Assumes the taxid values are Constants. (as in not formulas, but just strings.)
1. Install a button on your sub sheet. (Forms toolbar button will work fine for this.)
2. Assign this code to the button:
<font face=Courier New><SPAN style="color:#00007F">Sub</SPAN> CheckForDuplicateData()
<SPAN style="color:#007F00">'Button goes on sub sheet</SPAN>
<SPAN style="color:#00007F">Dim</SPAN> Dupe <SPAN style="color:#00007F">As</SPAN> Range
<SPAN style="color:#00007F">For</SPAN> <SPAN style="color:#00007F">Each</SPAN> taxid <SPAN style="color:#00007F">In</SPAN> ActiveSheet.Range("D2:D65536").SpecialCells(xlCellTypeConstants, 3)
<SPAN style="color:#00007F">Set</SPAN> Dupe = Sheets("main").Range("D2:D65536").Find(taxid, lookat:=xlContents)
<SPAN style="color:#00007F">If</SPAN> <SPAN style="color:#00007F">Not</SPAN> Dupe <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN>
MsgBox "TaxId '" & Dupe & "' already exists in the main sheet.", vbInformation, "Duplicate Data"
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">Next</SPAN> taxid
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN></FONT>
You would simply use this routine before copying to the main sheet. (That could even be included if there were no duplicates found...)
Hope it helps,
Dan
(Hey Nimrod, this any better than before?
)