Can anyone provide code to address the following issue:

Background
Via a control, my program imports records into an excel spreadsheet - cell
A1000 (the imported data actually populates A1000 through I1500). Previously
imported data already occupies cells A1:I100 (Column "I", of both sets of
data, contains a unique record key). The program then continues with
formatting, computing, etc.

Issue
What I need to address is, if any of the new data records match any of the
previously imported data, I want to abort the program (if any record in range
I1:I100 matches any record in I1000:I1500). This will prevent duplicate
records from being systematically input into the file.

Can anyone provide the code to address this situation?

Any and All Help Will Be Appreciated - Thanks In Advance!!!!