If you not only vote for one of the on-ballot candidates the normal way but also redundantly add that candidate on the write-in slot, haven't you spoiled your ballot? Yes, the intent may be obvious, but if you did something that stupid and that plainly invalid...

I strongly disagree. Any system, be it a voting booth, a DVD player, or an automobile, where the user indicates, “I want X, I want X,” and the system concludes the user doesn't want anything, is not a well-designed system.

Only from the perspective of a lazy designer or vote-counter is this user behavior “plainly invalid,” because the designer’s life is easier if users behave with complete predictability. Those “stupid” users! How dare they violate the designer’s rigid rules, clearly laid out in the small print!

But from the perspective of the user/voter, there is nothing inherently stupid or invalid about expressing the same choice twice—for emphasis, or redundancy, maybe. When I write a check I fill in the amount in words, then I write the amount again in numerals. This should make the check invalid? Even if the amounts agree?

The ballot is there to serve the voter, not vice versa. The clear intent of the voter should not be thrown out on a Pharisaic technicality.