As you are aware, a sim can only wear one pair of glasses at a time. When you clone glasses to make a new accessory, it will behave in the same way. This obviously isn't necessary if the accessory is, say, earrings - at that point you'd like your sim to be able to wear earrings *and* glasses, for instance.

The key is the the bin number in the Mesh Overlay XML.

Glasses use 0x0000000F

That's 0x (for Hex) followed by 8 digits.

Some of the other Maxis makeup uses other low numbers, but you can use any larger HEX number - you should try to make it unique to avoid conflicts with other accessory makers, although if you happen to have say, a 'hat' or 'earrings' made by other person in your game, you might code yours to use the same code - so that the game will automatically do the right thing.

You must be sure to change the bin number of all the ages you are using, but you only need to do it to your original BodyShop .package in your Saved Sims folder. Then if you later make recolors from it, BodyShop will include the new bin number in your recolors.

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What is a Hex number?

Hex numbers use 0-9 and A-F as their digits. They're written in SimPE as
0x followed by 8 digits.
So, the glasses value 0x0000000F has the numeric value of 15

What you need to do is create some random 8 digit long number in hex (using any combination of 0-9 A-F) and put that value in instead, after the 0x part.

Here's the math about Hex numbers, if you want to understand a bit more about why we're writing numbers with letters and numbers.
http://www.the-eggman.com/seminars/about_hex.html

(Thank you Dr Pixel and SnowStorm for these additional details)