Msgbox when cell contains certain value - code not working

I have the code below for a cell in my spreadsheet to popup a msg box when P3 has a certain value in it but it doesn't seem to be working (no debug or errors)

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The value in P3 is populated when another cell has a value in it and P3 is a sum of other cells (=sum(q3:x3)

I think this may have something to do with it but I don't know?

Any help appreciated, thanks.

Try this:

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I now have this code which does work but every time I click another cell it displays the message again, I only wish for the msgbox to pop up once. Can anyone help?

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