What happens if you tear your MCL twice?

Ligament retears. Whether or not one regains full knee function is determined by the severity of the tear as well as accompanying structural damage to other knee structures. Therefore a ist degree MCL tear torn without any other injured structures will heal well even if down the line one sustains another tear of the same magnitude.
Depends. What is the degree of tear and is there associated ligament injury such as acl or pcl? It is common to sprain the mcl of the knee. You want to be sure there is no other ligament or meniscal damage associated with this. Make sure you properly rehabbed your knee and this is the best way to avoid repeat injuries. A brace during activity may also help. More sever degrees of injury may require surgery.