Extra Hex feat will let you take another hex
1} Shaman's hex feature says "Unless otherwise noted, a shaman cannot select a hex more than once." Then, "witch hex" does not otherwise note. So this path to more than one witch hexes is closed, regardless of Extra Hex.
2} Extra Hex says "If you are a shaman, it must be a hex granted by your spirit rather than one from a wandering spirit." So this feat can't be used to access witch hexes either.
Bottom line: no.
I hate to dig up an old thread but I feel like this is something that should be brought to light;
The Shaman's hex class feature specifically dictates "Unless otherwise noted, a shaman cannot select a hex more than once."
However, none of their hexes come with that caveat. None. And since witch spells have the dictation "A witch cannot select an individual hex more than once," there is no reason the Shaman hex feature should say this.
Did the editors miss this in editing?
Sometimes the development team puts in language to future proof the abilities to account for possibilities in the future. For example, it's possible they introduce a hex in the future called 'Esoteric Lore' that lets the shaman add spells to their spell list. It might include language allowing it to be selected multiple times.
So no, there's no reason to assume they editors missed anything. You can pick one witch hex at present.