Enter key intermittently stops working on both my Macbook Pro's & external keyboard
It's very interesting that the external keyboard and internal keyboard are having issues with the same key. One action I also recommend when hardware like keyboards, back lighting, ports, chargers aren't working is to reset the SMC (System Management Controller). 

In the meantime, let us know a little more information:


Is the external keyboard wired or bluetooth?
Is this issue only happening with the Enter key, and no other keys?
Does the keyboard issue occur randomly, or have you noticed a pattern (like during certain apps, or when logging in, etc.)?
Do you have Apple Care or any other warranty?