Would it be weird if i told you that I have this one customer who I can't look in the eye because he looks almost exactly like my ex?
Wow, that's a crazy coincidence. Do they really look that alike?
He is a total doppelganger.
Still, you have to maintain a professional attitude. Just remember, while they may look alike, the customer is a unique person, completely unrelated to your ex.
I think I can handle this, it's a difficult situation but I'm going to ignore it.
Yeah, I'm sure you'll be able to handle it. Just treat him like any other customer, with courtesy and respect and don't let any feeling about your ex get in the way of that.