Message Box if Cell Text Matches Text in Data Table

[new123]

I have a "master" worksheet where a number of data items are tracked on each row, and several items are cross-referenced off other worksheets within the same workbook. I would like a message box to appear if text in column "I" matches text within another worksheet. For example, if I enter "ABC" in cell I4 and "ABC" is also in column A on "worksheet B", I would like a message to say "override C4"? Essentially, C4 on the "master" worksheet has a vlookup that inputs data base on what is entered in another cell, but it may need to be overridden if certain codes are entered in column I.