When installing a untried electric outlet and the outlet supplying electricity have the 4 terminal already used -- is it proper to add the latest circuit by installing the wires...

is it proper to add the latest circuit by installing the wires in the posterior of the outlet or is this overloading the electric outlet since the 4 terminals are human being used? also, is there any problem, if using a 12-2 rope on a 15 amp circuit? thanks for your relief.
How do I loosen really rusty bolts on...
Answers:    You need to remove the wires and verbs them together with one extra lead (6 inches) in respectively (black and white) group. Use wiring nuts or solder together and after tape. Now you own only one cable for each set of screw. That is the proper way to chain your sockets. It is called a "pig tail".The other can motivation many problems of intermittents and heat of the socket.
My ac dosen't turn on what could ...
you need an electrician or you may burn your house down.
the other things are what else is on that circuit and what is the nouns and distance theese things cant be just asked thats why an experienced electrician wishes to see first hand