Most people have the sensation they have done certain things before, but for some the feeling is constant.
Many of us occasionally feel we are repeating experiences
Trapped in their own 'Groundhog Day', they believe they have experienced unique events before.
A team at Leeds University was moved to investigate the phenomenon after seeing a patient convinced he had already been to his friend's funeral.
The researchers suggest the extreme deja vu is caused by a faulty memory process.
The patient who triggered the researchers' interest was referred to the memory clinic at Leeds University by his GP, but initially refused to attend because he was wrongly convinced he had been there before.
He even "remembered" specific details about times and places he had met the doctor at the clinic.
The man, who was in his 80s and had dementia, also described how all the programmes on TV had been on before, and - perhaps more unusually - that he could hear the same bird singing the same song in the same tree every time he went out.
Memory circuit failure
Publicity about the case led around 10 other sufferers of the condition to come forward - many of whom were younger than the initial case.
However, all had conditions such as head injury or chronic pain which the researchers believe is linked to the deja vu.
Dr Chris Moulin, who is leading the research, said: "It is a new symptom, not a syndrome."
But he said it could significantly affect sufferers' lives.
He said chronic deja vu sufferers were overwhelmed by the sensation, even causing depression in some.
Dr Moulin hopes studying people with the condition will help understand memory.
"We think the symptom is caused by a failure in the temporal lobe, where a circuit which is fired up when someone remembers something is malfunctioning".
He suggested that, in a person with chronic deja vu, this circuit is either overactive or permanently switched on, creating memories where none exist.
Dr Moulin added: "Chronic deja vu sufferers need the reassurance that they're not alone and we need them to help us learn more about who has it, what causes it, and why."