This sounds almost like inputs on the back of some TVs that were meant to accept the output of a mono VCR and if just one jack is connected the sound comes out both sides. But I have never heard of anything like that in a phono input.
If you do not have another receiver with a phono stage to try the turntable on, you could plug it into one of the other inputs on your receiver, such as AUX or Video. The volume will be very low but you should be able to hear something and determine of there is some kind of problem going on with the Phono input alone. Do not plug anything else into the Phono input though, it would be very loud and could cause problems.
Let us know what happens, it may just take process of elimination to get to the bottom of this mystery.