I know this is probably the last argument most people want to hear, but is this not a case where more independent implementations would result in less risk?
No. This is nonsense that has been pushed by those actively trying to co-opt the network
Slightly OT, but can a consensus based system ever
really be co-opted, though? Everyone seems to have their own slightly different ideas of what makes Bitcoin what it is. If you find yourself on a minority fork, it's because you aren't following consensus. It's the numbers that matter, not what any one person believes Bitcoin "
should" be. Even though we might disagree with them, many BCH users will argue that Bitcoin has already been co-opted, but the simple fact remains they don't have the numbers behind them to do anything with that assertion. So they have to settle for being an altcoin. That's just how it is. A day may come where you find yourself on the wrong side of consensus. If that day comes, you would then find yourself deciding whether it's more important to stick with what you think it should be, or to accept it for what it is. Maybe that's all getting a bit philosophical, though.
Back to the main purpose of the thread, though. Yes, there are definitely some issues with multiple implementations if it's done in the wrong way. It seems there's no simple answer to this one. Aside from the things gmaxwell and achow101 mentioned, I suspect one of the primary flaws with multiple implementations is that much of the code would simply be copied from other implementations anyway. It wouldn't necessarily ensure catching any present faults, even if people were taking the effort to run two different clients to compare results. If they've inadvertently duplicated the bug, it won't make any difference. Much like how any of the altcoins that may have been affected didn't spot duplicate inputs either.