What would you call this "double entry" principle in English?

Solution 1:

I think "double-checked" or "cross-checked" would be more accurate than "redundant" here. If the intention was that the system was still usable even if one of the computers fails altogether, that would be a redundant system, but (I assume) it isn't; the cross-checking that both operators agree on the numbers is a necessary part of the system if we want to detect errors.

"cross-checked" is perhaps more precise; the point is to have two independent sources to check against each other, whereas "double-checked" could be a single source being looked at twice. (Though ultimately there's one source, the actual ball with the number.)

Compare cross-check and double-check.

(The next stage would be to have three operators, and to accept results if two out of three agree. Then the third computer is redundant for error detection, but not for error correction.)

Solution 2:

In engineering, we would call it a redundant system.

redundancy: (1b) (engineering) The inclusion of extra components that are not strictly necessary to functioning, in case of failure in other components

{ODO}