Without my bringing any of the usual jokes into this, I’d like to suggest redundant, based on the Latin for "overflow."

We hear a lot about redundant systems these days, especially as applies to safety.  One of my subcontractors is currently adding a redundant system to one of their larger pieces of equipment after a QA review determined that one system could fail and yet leave the machine operable.  The new setup will double up on the safety system, and also ensure a shutdown if both safety companents fail.

A word with two connected, yet distinct meanings.

Historically, or at least traditionally, redundant meant ‘superfluous’ (also from a root meaning ‘to flow’). Yet today, in computer and certain industrial uses, it means ‘duplication or repetition of elements in electronic equipment to provide alternative functional channels in case of failure’ (AHD).

Hmm. What about ‘reiterated redundant repetition’?  ;)

So the redundant systems are superfluous until something goes wrong, when they become essential (I was going to use unprescindable, but I might be making this word up, transliterating).

Prescindable is a perfectly good word formation, hinted at in the Suggest a Word/Schism thread.

Prescind, according to the AHD, means to separate or divide in thought; to consider individually, or to withdraw one’s attention. It comes from Latin praescindere, to cut off in front : prae-, pre- + scindere, to cut off, split; see skei- in Indo-European roots.]

My admittedly scant Spanish language resources show prescindir de as meaning ‘to do without,’ an interesting sense development from the Latin.

So yes, we have arrived at a situation where ‘redundant’ means ‘essential.’

The line to the Tower of Babel forms to the right.

When I hear the word redundant it makes me think of people who say "please RSVP" (please respond, please) or ATM machine.   ::)