IG doesn't mean dedicated ground for each outlet. As Iwire says, check your specifications. Generally it means a separate isolated and insulated grounding wire besides the equipment grounding conductor.
So, for a 120V outlet, one would end up with a hot, a neutral, an equipment ground (for the conduit & outlet box), and an isolated ground (for the ground prong of a receptacle).
You can usually string several IG outlets together without a problem, depending on specifications. However, some engineers and/or specifiers have different ideas about what IG means.
Lastly, if the breaker panel feeding your circuit does not have an IG bus with an upstream isolated ground conductor back to the main, are you really getting an isloated ground?