Can't remember if I've posted this one before, so apologies if I have.
Man goes to his GP and says that in both social and work situations he feels compelled to loudly sing 'The Green Green Grass of Home', and it is becoming both embarrassing and intrusive.
The GP tells him not to worry, he know what the problem is, and that he has Tom Jones Syndrome.
The man is relieved and asks if the condition is quite common.
The GP jumps on his desk and loudly sings 'It's Not Unusual'.