Serial Number should be unique

[caseywatts]

We should not be able to put in two equipment items with the same serial number

Not sure what the code in the model says but could be something like:
validates_uniqueness_of :serialnumber

Ideally different models can have the same serial number (in case that happens: camera x and laptop y serial numbers are the same), but that's rare. It's okay short-term to have it unique across all items regardless of model.