Answer

My answer.md

There's multiple issues:

• somethingIsTrue ? doSomething(); is not valid in JavaScript. See MDN on ternary operator
• Frankly, I don't think Array.prototype.reduce is the right choice. Reduce creates something from all array members. I mean you could use it for this, but the code would be unnecesarily obscure. For generic looping, use either map (1:1 conversion of array elements) or forEach

I'd use forEach, or just classic for:

var d_loc = [{'Common Name': "Foo"}, {'Common Name':"Bar"}, {'Common Name': "Foo"}];
const accounts = [];
const result = d_loc.forEach( (userInfo) => { 
    const name = userInfo['Common Name'];
    if(accounts.indexOf(name)<0)
        accounts.push(name) 
    } ,
[] )

console.log(accounts);