Why do I get a 'double tone' when playing top 'G' - Saxophone

Hi, I've had my tenor Yani902T in bronze for about 2 years, but have only been playing for 4. The problem I have is that when I play 'G' with octave key, more often than not, I get an #F undertone. I can cure this with a shed load of diaphragm support but still need to squeeze the reed a little more especially if playing p, pp etc.. The rest of the notes from low B flat up to top #F are totally unaffected. When first purchased, this problem wasn't apparent and only occured over a period of 2/3 months. I decided to have it serviced professionally and was amazed that the undertone had gone! Unfortunately it returned about a month later with a gradual decline in G quality over a 30 minute practice. It's getting to the stage where I fear playing that one note! Stupid for a 50 year old!
Any help would be appreciated.

Regards Hal