Uniqueness constraints for Entities?

[Chr1sG]

Unique data fields

In our customer management system we assign customers unique ID derived from our accounting. I have created a field for this, and already made a misstep of creating a duplicate customer as I could not find them in my list at first. This is especially handy if creation and data population is done manually. Something like a check-box to check for a field being unique would be nice.