You can never have too many:
 

I'll start: extension cords. Of various types and lengths. Now you--you can never have too many:

You can never have too many wax earplugs.

You will lose half-full boxes.
Your cat will chew the ones you left in reach because 5 years is not enough to teach you that lesson.
The one time you are absolutely, completely, nowhere-else-to-look out of earplugs, your parents will have a horrible row that you really did not need to hear as you were mentioned in it and you can't forget it even though they both have the nxt day.

You can never have too many bungee cords. I love them for stowing the camping equipment in the back of my truck; they keep boxes and containers closed that won't stay shut; their uses are endless. And let's not forget bungee jumping!

backups
prescription refills
bytes of speed in your internet access
hand tools

clamps

dollars
choices (so long as you actually choose)
imagination (many/much)

Scissors - I try to keep on in every room because I hate hunting around for them

Reading glasses - same strategy as scissors, because no matter where I put a pair down, I can never find them

Backup pairs of sunglasses.

Mobile phone chargers

I went so far as to buy two extra pairs of frames, online, for my prescription glasses with magnetic clip-on shades. Just for the sunglasses part, which I lept losing. I lost the last pair of shades, into a lake, last summer.

So I went to get a pair of Transitions lenses for my recent vacation. Guess what? Those take at least 10 days to be shipped in from an external laboratory. What about the prescription, stand-alone sunglasses? Sorry, those aren't done in-house either. Lenses have to be cut at the manufacturer.

News flash: you can't just walk in and buy ANY KIND of prescription sunglasses. Don't believe the commercials.

I ended up getting $20 sunglasses at Walmart that go OVER my regular glasses. Those worked great, until day 2 of my drive back home, when the earpiece just fell off. The plastic failed after about two weeks of moderate use.

goggles

kisses

eyeglasses
underwear

Mine aren't prescription, but I have no fewer than 5 backup pairs in my closet. They're all identical--I am very picky about my sunglasses, so when I found the ones I liked, I bought them all. And then I have the on-the-spot backup pair that lives in my purse, for the time between when I break/lose my real ones and when I arrive back at my house. These are far from the ideal size/shape, but they are made of thick plastic and very sturdy, so they will never get broken just by being carried around.