Stress is always written and only occurs once per word.

It never once occurred to him that he might die.

These occur once per year and usually run over the weekends.

Such things occurred only once in a life- time, if that often, and he had thrown his chance away.

Even for a very long text, about half of the words found will only occur once.

These concerts occurred once a month for six years beginning in 1998.

She felt stupid now, but it had never once occurred to her.

This is often the case with words that only occur once in a text.

It never once occurred to her that there was anything wrong in this.

Once that occurred, they never went into the central chamber.