Do people with schizophrenia have one visual hallucination that they usually see?

Not usually. Hallucinations are auditory or visual with auditory more common. Auditory may be vague and benign or clear and foul and threatening, often demanding a person to say or do threatening or harmful things like hurt or kill others or even themselves. Visual hallucinations are more rare and can be repetitive. In my experience having only one visual hallucination is very unlikely.
Hallucinations. Auditory hallucinations are more common. Visual can suggest organic factors such as toxicity. Consult your prescribing doctor for workup.
Auditory. Schizophrenics usually suffer predominantly from auditory hallucinations. Not so much visual, gustatory, tactile, or olfactory. Occasionally, however, those who do have visual hallucinations will report seeing the same one repeatedly. However, this is not always the case.
No. As stated visual hallucinations can vary but similar themes might recur.