知らされる means "to be informed".
知らせる means "to inform"
Why is 知らせされる not used? I almost always hear/see 知らされる。Is it a different verb entirely?
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There is also a verb 知らす "to inform/notify" and 知らされる is the passive of this verb.
As explained in @l'électeur's answer to the question Causative せる/させる vs. す/さす, 知らす is an older form (文語体) of 知らせる (口語体) with the same meaning. (Most if not all causative forms ～せる can be thought of having derived from an older ～す verb.)
You also ask about *知らせされる, which is not a correct form. The causative of 知らせる is 知らせさせる. The passive is 知らせられる.
知らせる (and 知らす) are already the causative form of 知る, even if they are used more idiomatically as "to inform", rather than the literal "to make [someone] know".
In any case, the causative of 知らせる, which would be 知らせさせる, could in principle be used, e.g.
Please allow me to inform [someone else]
However, such cases are probably quite rare.
As you seem to have noticed, 知らせられる and 知らされる are passives of two different but related verbs that mean the same thing.
I think 知らされる can be thought of as the de facto passive of 知らせる. 知らせられる appears not to be used. (At least not as a passive; of course, the potential is of the same form.)
Something similar seems to happen with other verbs. For example, やる has a causative やらせる and de facto passive やらされる. The passive やらせられる is hardly ever used.