Checksum for equivalent cells in a table?

I have an Excel table that looks like this. In this, equivalent cells should always equal each other - ie both Mary/Alex cells equal 8, which is correct.

What I want is some sort of checksum which can highlight when two cells that are supposed to be equal aren't, such as looking like this (not picky about appearance here). Importantly, I also need for it to be easily expandable for additional rows and columns - the actual table has over 100 rows, with more being added routinely. If it was smaller I could just brute force it with a bunch of =IF statements, but that's not feasible for this.

Desktop Excel 365, intermediate knowledge

Thanks.