Peter, is your question about harmonic balance?

Western harmony generally uses Tonic (1), Mediant (III) and Dominant (V) to make a triad. Notice this is going up in thirds all the time, so you can also compound the VII, II (9th to you and me - it cycles round the octave), IV (11th) and VI (13th).

Is this about defining if C E G B is I 7th or iii 13th?

Or is it a question about balance of doublings within the chord? For basic harmony, avoid doubling thirds because it sounds weak, but what happens if you're writing for orchestra? Flute and piccolo play in octaves very often, so technically, the melody is often "doubled" - probably best to think of it in terms of a single combined timbre.

Or am I reading too much into this?

Hope it's some help, cheers, Jon