I bought a GE GSS20IEPH Side by Side Refrigerator in 2005, every 2 years almost to the date I have had to replace the heating element that prevents the back of the freezer from frossing over and stop cooling. I am now replacing my 3rd heater in 6 years, is this normal, could something else be wrong?

No shouldn't have to do this,are you installing part WR51X10101 heater?if so have you ever had to replace the motor board?do this when you're in the freezer go on top of the evaporator coil,look at the sensor or thermistor same thing,it looks like a white bullet part wr55x10025,look at it,if it has a black ring around it you have to change it,ge had problems with these thermistors,also everything on this fridge runs through the mother board open up the back cover and look at the board if you don't have 7 black relays in the board you have to get the newest board in there,this could be taking out your heater, board part wr55x10942p go here look at picture,if you don't have all the relays on left bottom side get a new board if sensor has black ring replace that too.i've never had to replace a heater again on a fridge and not that soon,make sure no wires are shorting out anywhere that could take out the heater
https://www.repairclinic.com/PartDetail/Control-Board/WR55X10942P/4512753?replacedManufacturerPartNumber=wr55x10942
also i have a diagram of the board it shows you what's on the board if you look at j11 and j9 these are the connections for the defrost heater,the j8 is for compressor so if you jump out j9 and j11 the heater will come on and no compressor with the gray wire connector removed from the board.
https://www.flickr.com/photos/zenzoidman/4482598198/sizes/o/in/photostream/
and i have another site that shows you everything about the board that's very helpful
https://www.scribd.com/document/243353634/GE-Electronic-Refrigerator-AC-Diagnostics-Manual
if your board is the older board i would call ge first,tell them what's been happening and i've seen them send parts to people for free or short money,it's worth a call all they can say is no we can't help you or they could help you out, i'd call.can you do me a favor i've answered a few peoples questions and haven't heard back from them only like 2 people out of like 50 can you just let me know you got this and let me know how you make out thanks and i hope this helps you out.that ge diagnostics manual shows you alot of good stuff.make sure all the wires are in good shape that go to the heater,change out the thermistor also there's a t-stat on top of coil i would change out the t-stat and thermistor if something is bad it could be keeping the heater on longer than it should be and short living it,any wire connections that you have cover them with sealant if you have wire connectors anywhere in there put sealant in there so no water can get into wires and check the board.good luck hope this helps you.