Coda allows multiple codes with the same value

[as2388]

I can't think of a good reason to support this. I think when someone attempts to save a scheme with multiple codes, we should prevent this and inform them that that code already exists.

It's also interesting to consider why this is happening. I think it gets quite hard to find codes when there are a large number of them. We should look into this too (e.g. perhaps we should display codes alphabetically?)