BTW - This is actually no simple task. I do a fair amount of data scrubbing for my job and it can definitely be overwhelming. You need data standardization. Are they using emails, names, phone numbers or addresses as the unique identifier? Then, to flag an identifier, you will generally need an exact match - meaning if you have an extra space, period, abbreviation, etc., there will be no match. On top of that, the standardization needs to be the same across all lists (including the ones you are comparing to). Consider the sheer volume of names being compared across the world and in many languages. Have you ever had an Excel file open with 15,000+ rows and multiple columns and tabs? It's not easy!
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