Why do some people wear single hole tzitzit and others double?
Chassidim do many things differently- maybe one of them reading the question will give you some insight.
Chassidim have 2 holes. This is because the tzitzis strings have to hang over the corner of the begged, not hang down – which is what happens when you have 1 hole.
I wear 2-hole tzitzis (for the reason mentioned), and I’m not chassidish.
there is a shita of the baal ha’ittur that tzitzis needs two holes. As you will recall, the gemarah tells us that kesher elyon is deoraysa, because otherwise the tzitzis is unattatched. According to the baal haittur there is a in-and-out even without a knot, which constitutes an attachment regarding shabbos. This is one of the reasons other rishonim argue with the baal haittur. As mentioned, chassidim generally follow this shita and litvaks do not.
I think it is more kabilistic than halachic
Even chasidim have one hole on the tallis gadol.
does anyone know of any exceptions
Ubiq: they have one hole on the Tallis gadolinium to avoid appearing “holier than thou”. (Pun intended)
*talis gadol (thanks spell check for your running commentary)
FYI gadolinium is a rare earth element named after the Finnish chemist Johan Gadolin.