In that case, you wasted an exploit for no gain. The problem is this:
1. Unlettered arcanist, as a hybrid witch/sorcerer, has the witch's familiar. This functions like arcane bond (familiar), with the exception that it also has powers related to its role as a living spell-book.
Witch class description:
Witch’s Familiar (Ex): At 1st level, a witch forms a close bond with a familiar, a creature that teaches her magic and helps to guide her along her path. Familiars also aid a witch by granting her skill bonuses, additional spells, and help with some types of magic. This functions like the wizard’s arcane bond class feature, except as noted in the Witch’s Familiar section.2. The familiar exploit gives you arcane bond (familiar):
An arcanist with this exploit can acquire a familiar as the arcane bond wizard class feature, using her arcanist level as her wizard level to determine any of the statistics and abilities of the familiar. If the arcanist receives a familiar from another class, her levels of arcanist stack with the levels from that class when determining the familiar’s statistics and abilities (this ability does not stack with a familiar gained through the bloodline development exploit; she must choose one or the other).
The last line refers to the case in which you have familiar and then take bloodline development (arcane,) in which case the bloodline would grant neither a familiar (by the familiar exploit's rule) nor the item bond (by the bloodline power's rule.)
It would take a rather unusual case (such as gaining a Tumor Familiar - available to alchemists via discovery and Aberrant sorcerers via feat) to be able to get a second familiar. Most paths of getting them either function as arcane bond (which nixes it) or explicitly disallows multiple (eg, the wasp "familiar" that Calistria can grant worshippers via a feat, or the familiar gained through the Familiar Bond feat chain.)
Getting access to a witch's familiar, and then a wizard's familiar, is definitely NOT one of the unusual cases, any more than it would be if you were a 3rd level witch / VMC-wizard.
the same as if your'e a vanilla alchemist who pick the Mutagen discovery. you don't get 2 of the same class ability. the archtype already give you the arance bound class (familiar) ability. can't pick it again with an exploit
(Mutagen (Su) (Ultimate Magic pg. 16): This discovery gives the alchemist the mutagen class ability. (This discovery exists so alchemist archetypes who have variant mutagens, such as the mindchemist, can learn how to make standard mutagens.))
The default is that if you have the same ability from two different classes, then levels in those classes don't stack, but you have them separately.
https://paizo.com/paizo/faq/v5748nruor1fm#v5748eaic9o80
unless an ability specifically says it stacks with similar abilities (such as an assassin's sneak attack), or adds in some way based on the character's total class levels (such as improved uncanny dodge), the abilities don't stack and you have to use them separately.
i think i remember a dev or faq that said you can’t count one character level twice for the same class feature; but maybe that’s unofficial and/or something i made up.
It gets quoted a lot, but I think that's just because you can't count a stat more than once for the same bonus, like no double dipping charisma to damage.
As far as I know there is no official or even semi-official source saying that you can't count the same character level for the same class feature from different sources.
Levels of different classes that are entitled to familiars stack for the purpose of determining any familiar abilities that depend on the master’s level.
No, you cannot gain two familiars from two different classes, so multiclassing Wizard + Alchemist (Tumor Familiar), Sorcerer (Arcana Bloodline), or Magus (Familiar Arcana) wouldn't grant you 4 familiars. These levels would simply stack for the purpose of allowing your familiar to level up.
But if you have two familiars from the same class, I do believe you would actually get two familiars.
Levels of different classes that are entitled to familiars stack for the purpose of determining any familiar abilities that depend on the master’s level.
No, you cannot gain two familiars from two different classes, so multiclassing Wizard + Alchemist (Tumor Familiar), Sorcerer (Arcana Bloodline), or Magus (Familiar Arcana) wouldn't grant you 4 familiars. These levels would simply stack for the purpose of allowing your familiar to level up.
But if you have two familiars from the same class, I do believe you would actually get two familiars.
So would a dual-archetyped blood arcanist (arcane) / unlettered arcanist, who then took the familiar exploit, get three familiars?
I don't agree that it allows more than one. Let's consider this.
1. The familiar exploit gives you a familiar as arcane bond.
2. The unlettered archetype gives you a witch's familiar, and a witch's familiar operates as arcane bond except that it also has spell-book powers.
3. The arcane bloodline's L1 power is arcane bond.
If these were spread among three classes (witch / arcanist / arcane sorcerer - please be kind to your caster and don't do this, by the way) - what happens would be clear: you'd have a witch's familiar, at a level based on witch and sorcerer, and arcanist IF you have the exploit.
This doesn't change just because the arcanist can use archetypes to pretend to be a witch and sorcerer! Thus, you would have a witch's familiar. The sources' levels - all being your arcanist level - cannot stack ABOVE your arcanist level; thus, your familiar is = your arcanist level, and you have two dead features (the bloodline power, and the exploit.)
I want to say it does nothing... but the simple fact that the familiar exploit calls out the possibility of having a familiar already through bloodline development in a way that doesn’t prohibit having both... I am inclined to say that the familiar exploit is one of the only ways out there to have two familiars... and for an unlettered Arcanist that can be quite valuable... your witch’s familiar chills out within reach of you, while your exploit familiar (which btw is able to be a figment familiar unlike the witch’s familiar) runs about taking risks you’d never subject your witch’s familiar to for fear of losing your spells.
The only other familiar granting ability I know of that allows having multiple familiars is the Occultist’s soulbound puppet... and that just grants a temporary familiar as a tiny construct...
I mean, duplicate familiar is a thing, so a wizardwizard can wander around with a dozen or so full BAB (spirit binder) familiars Manyshotting everything that moves all day, so an Arcanist being able to get two familiars by throwing all his class features wouldn’t be very impressive. I don’t think the Blood Arcanist thing would get you a third tho, because the Arcane bloodline’s familiar stacking language would supersede and it isn’t restricted to ‘familiar from a different class’.
If you actually read arcane bloodlines familiar stacking rules, it actually only prohibits getting a bonded object with a familiar. And since it is the first bloodline power, it already doesn’t stack with bloodline familiar. The multiple classes granting familiar rules do the rest in preventing an arcane bloodline sorcerer from having multiple familiars... though a Crossblooded arcane and serpentine bloodline sorcerer might be capable of having two familiars since the second bloodline power for serpentine grants the snake familiar (using level-2 for its progression).
Duplicate familiar only grants temporary familiars, with the exact stats and abilities of your original, which is hardly game breaking. Having multiple permanent familiars, each capable of having different archetypes and creature types can give a character a significant boost in power and versatility. Which is most likely why the soulbound puppet is temporary, and the familiar exploit explicitly states that you must choose one to stack with other class levels if you get one from another Arcanist ability like bloodline development. Such restrictions prevent having an army of full strength familiars of different archetypes from multiple sources.