Macro to check and clean double data entry??

Hi, I hope someone can help me on here... I work for an NGO in Africa collecting fish catch data and the data is entered by various people. to avoid errors we have been doing double data entry but checking for errors using simple IF-Functions and conditional formatting in excel is particularly time consuming as it involves constantly switching between worksheets. What I need is a quick and effective method of checking, verifying data and producing a clean data sheet.

So to outline the data

I have two worksheets data1 and data2 (upto 20 columns and possibly thousands of rows) both data is the same but has been entered by two different people. In a third worksheet I want to have checked and cleaned data. What I am looking for is if it was possible for a macro or similar to run through the two worksheets and where data is the same paste it directly into data3 BUT (this is where it gets tricky and far beyond my level of understanding of VBA) throw up a message box that gives the person checking the data three options ...
option 1 the value in data1 is true
option2 the value in data2 is true or
option 3 neither 1 or 2 is true and new value is recorded.
The option chosen would then be entered into data3.

I have uploaded an example of the kind of data I am looking at - where data3 currently reads ERROR would be where I need the macro to provide the checker with a choice box?

I have read a number of threads in search of the answer but none of them yet seem to give me what I want!I have every faith in the people on this network to be able to help me with this problem

Fingers crossed and thanks to anyone who can assist!