I'm studying a piece by palestrina (Agnus Dei is the title of this piece, but I'm not sure of the name of the work) and in this 3rd bar we have a sharp in the parenthesis and a natural over the B, which from the key signature is flat. I'm pretty sure that it's supposed to be played as B natural, but would the editor include the sharp as well as the natural accidental?
In Palestrina's time, the "rules" for writing accidentals were different from the current conventions.
Even as late as CPE Bach, sharps and flats were sometimes used to change the pitch of a note by a semitone up or down, not to indicate the absolute pitch as they are today. So the sharp in front of a B flat would actually mean "B natural".
Also, the rule that "accidentals remain in force to the end of the current bar" was not strictly followed - especially since at this period, bar lines were often placed in an irregular pattern and the modern concept of "time signatures" did not yet exist.
In your example, it was probably "obvious" to Palestrina's contemporaries that the second B in the word "mun-di" should be the same pitch as the first one, and they didn't bother to notate things that were obvious! In fact, writing a natural before the second B could have been read as returning to the pitch in the key signature (i.e. B flat), and writing another sharp could have mean raising the pitch by another semitone (i.e. from the previous B natural to B sharp, which clearly doesn't make any musical sense) so they didn't really have a style of notation that matches the modern style of writing accidentals.
In some early notation, it was generally assumed that in scale passages in minor keys, the "melodic minor" was intended when no accidentals were written - so the notes E F G A G F E with no accidentals might mean E F# G# A G-natural F-natural E. Since modern performers have inevitably heard a lot of tonal music written after this period, it can often be hard to decide what pitches the composer originally intended. One valuable source is contemporary transcriptions made for instruments that used tablature notation (e.g. the lute) where there is no ambiguity about the pitch of the notes - at least, so long as it is clear what tuning system was being used!
This rather careless (by modern standards) notation resulted in some long-standing mistakes when "early music" was being rediscovered in the 19th century - including some in J S Bach.
The small naturals above the stave were added by the editor to show the modern notation for the accidentals. Similarly, the parentheses ( ) round the sharp were added by the editor to show that this was the original notation, but it doesn't have the same meaning as it does today.
The original music had a sharp printed as an accidental (that's why it's in full size). Which is a quaint way of doing a B natural. The editor(?) put it in parentheses (though they should have been small-sized then) and placed the "musica ficta" natural over the note (small size, so usually added after the fact).