Finding Character Duplicates In Two Lists

I have two lists of numbers.

The first list contains site numbers of people who havent responded to me.

The second list is the master list of site numbers along with a column showing the date they responded.

Now, a site number is built like this:

123456/0001
123456/0002

So it is possible for the same 6 digits to appear more than once in the master list.

What I need to do is to compare the first 6 digits in the non responder list against the master list, because some sites, like the example above, may have more than one '0001' tag and so if they have responded to me from site '0002' I dont want to spam their other sites with emails.

I've tried using match and various formulas I've found from google etc, but nothing seems to work!

The goal of this is to get a list of non respondents that have not responded from any of their sites listed in the master list.

Non Responses

Master List
Date of Response

203289/0001

201162/0003
06/06/12

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