If a Half-dragon Drow takes the "Surface Infiltrator" Alt. Race Trait. Does he still have Darkvision. I only ask because he has darkvision from being drow and he still has darkvision for being a half-dragon.
From what I understand...
When you receive the same racial ability from multiple sources you take the best of them (e.g. you would get darkvision 120 ft from the drow as its higher than the half-dragons 60 ft.).
Because you take the best of the Darkvision sources you only get one instance of Darkvision. So if a trait would cause you to loose darkvision you just wouldn't have it at all.
I would like someone else to weigh in on this to make sure while I search for sources but I'm fairly certain that's how it works.
From what I've always seen, applying templates is a step by step process starting with a base race, then inherited templates, then finally acquired templates.
In this instance, you would select all decisions for the base race first, losing your Darkvision 120, then gain the template granting you darkvision 60.
I re-read your question, my bad.
To be clear, you don't EVER have 2 of the same racial ability, you only get the best.
You would qualify for the 60 ft darkvision as your not getting darkvision from being a drow. You also do not get light blindness because of Surface Infiltrator. You have low-light vision too. ;)
Edited for clarity.