What happens when the contingency is not met

[Rahul Gupta]

I am wondering what happens if i try to sell my primary residence house  and get another primary residence (Let's say a duplex) while i have a mortgage on the first one. 

From what i understand, i can have a contingency clause while buying, which says that i will only be able to complete the transaction if my first house sells. What happens if the first house sells and some reason, the financing falls through before closing on the one i was trying to buy(the duplex)? 

How to make sure that we don't lose the house we currently have in the process and not be able to get the new one for some reason.

Thank you