For most instruments, you can only play one note at a time. Hence, being a melodic instrument. Others, like keyboard instruments allow you to play two or more notes at the same time. However, they must be different pitches as keyboard instruments have one key for each respective pitch.
Guitars on the other hand have the ability to play two of the same pitch at the same time. For example, the open high E string can be played with the B string fretted at the 5th fret – producing an E in the same octave as well.
While they be noted the same, they will slightly differ by a few frequencies in pitch causing an interesting harmony.
Consider this as you are writing for instruments like the guitar where you are able to double the same pitch using different strings.
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