Salesforce Junction Objects

Question

I have my contacts and a custom object named programs. I created a junction object using to master detail relationships with contacts and programs. I want to avoid relating the same contact to the same program. I tried triggers but I couldn't create the testing part to use it outside sandbox.

I went back to the basics and created a Unique text field. I tried to use default value but EVERYTHING i write in that crap is wrong -_-. I tried Contact__r.Email & "-" & Program__r.Name but to no avail.

I tried workflow rules with a field update but my field update NEVER runs.(Yes I did activate the workflow rule) and I didn't know what to write in my rule's code.