Why in this one recipe parsley is used in 2 different ways?
Parsley is both a flavoring herb and a decorative element for plating the finished dish.  By adding it during the cooking, it imparts flavor to the shrimp.  By adding it to the finished dish, it provides color and flavor.  The cooked parsley will have a slightly different taste to it than the fresh parsley.  This is how cooks layer the flavors in a dish.

TB stands for Tablespoon.