In many ways Latour is thoroughly disingenuous in terms of his intellectual predecessors. He is an enthusiastic cheerleader for the relatively unknown or unfashionable - Tarde, Serres, James, Dewey, etc. - but either ignores or distances himself from other more hegemonic figures whose influence on his work is, nevertheless, unmistakeable. Derrida, Foucault and Deleuze are the poststructuralist/postmodernist trinity that he attempts to distance himself from perhaps above all. He generally has good things to say about them when prompted and will occasionally cite Foucault as a good example of this or that but in general they either sit in the background or, in the case of Derrida, become straw men for Latour's rhetorical bonfire.
So, Latour as a philosopher is thoroughly precedented and this is obvious despite his best Oedipal efforts to distance himself from many of his predecessors. However, those critics who say 'this argument is very similar to that made by X, therefore X said it all before' are utterly misguided. For one thing, the originality of the likes of Foucault and Derrida is itself often overstated. Derrida didn't invent the notion of a regulative ideal that must be pursued despite being unachievable (but whenever anyone adopts such a position now they are said to be Derridean). Many things are often labelled 'Deleuzian' that could just as easy be called Nietzschean. Likewise Foucault and, say, Bachelard (or Marx who Foucault delights in not citing).
Derrida/Foucault/Deleuze are the presently hegemonic generation of dead white men who are the most read and so ideas tend to get attributed to them rather than to anyone else. The innocuous academic phrase 'as Y has argued...' inevitably suggests that Y is the brilliant originator of that argument. It is the enduringly critical role of intellectual history to stretch out these inelastic trajectories into proper genealogies (to whom one may attribute that insight, I'll leave that question open!).
But there is a more fundamental and important flaw in the 'X said it all before' attitude. It misses the fact that subtle mutations can make huge differences. Yes, there's hardly a page of anything Latour has written that can't be traced back to a whole slew of predecessors. But taken as a whole just how much of a resemblance does his work bear to even his closest progenitors - e.g. Deleuze? Not much. One has to be attuned to the differences, perhaps, but differences there are - and many of them.
The mutation that turns the unstable linguistic structures of Derrida et al. into the actor-networks of ANT is at once a small difference and a huge one (isn't this inversion of scale a classic ANT insight? or is it?!). Likewise, the extension of actors into objects in the manner of Harman et al. - a small mutation (almost undetectable at first) that leads to a completely different result.
In a sense Latour's philosophy is just poststructuralism 'writ large' - but he had to write millions of carefully translative, transformative words in order to achieve that!