Duads are actually the zodiac projected on one sign.
The idea behind it is that a planet on 5 degree of a certain sign is behaving differently than one on 25 for example.
So the ancients treated one sign as a whole zodiac, giving 2,5 degrees to every duad sign.Of course it doesn`t really change your main signplacement, it is said to add a subtle flavour to it.
For example:
I have Venus in Capricorn but in the Pisces Duad. So my earthy Venus gets a bit of dreamy quality through Pisces. It does make sense.
Another theory is those of "overtones" or signrulers, also adding another laying. This is actually something I can see very clearly in my own chart.
Example:
My Sun is in Sagittarius,
the ruler of my Sun (Jupiter) is in Pisces, which gives my Sag-Sun a little bit of watery quality.
I was always surprised when I learned that I have practically no water element in my personal planets; this just didn`t seem true.
I was nothing of that typical energetic dynamic fiery personality I should be with Sun, Mercury, Mars, ASC in SAgittarius.
Interestingly all my overtones / ruler of personal planets are in watersigns.
Sun, Mercury, Mars, ASC in Sagittarius with ruler in Pisces.
Moon in Aquarius with Uranus in Scorpio / Saturn in Cancer
Venus in Capricorn with Saturn in Cancer.
Just two theories to try out.