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In major and minor triads it is best to double the root of the triad. In these triads the fifth may be doubled or omitted. Avoid doubling the third of a major triad ... but the third may be sometimes be doubled in a minor triad. In a diminished triad double the third. In an augmented triad double the root or third.
Taken from- http://members.nbci.com/tgue1451/checkp.html