Accidentals change the note pitch by ½ step.
A sharp raises the pitch a ½ step.
A flat lower the pitch 1.2 step.

If you have measure with the same note only the first note must have the sharp. The other note inherit the previous sharp accidental. In other words once a sharp or a flat has been added to a note that note is sharp or flat for the entire measure.

If you do not want the sharp note you use a natural to cancel the sharp accidental and now the note is natural for the rest of the measure.