Can multiple overt/covert intentions produce identical EEG readings?
To curtly answer your question: given the sheer amount of different thoughts possible (billions I guess) and the limited spatial resolution of the standard EEG as well as the restricted number of characteristic parameters to describe EEGs (frequency, amplitude, latency) it is safe to say EEG is not suitable to characterize all of the brains processes. In fact, no state-of-the-art technique can do so, as far as I can see.