Arrow-Debreu and Pareto Optimality

I'm not sure if this forum these days still is active. But anyway, I've found something interesting:

Go to MWG page 694. The idea that after the state of the world is revealed there is no further trade appears a little bit awkward to me.

Assume we have two consumers, two states, one good. w_1 = (1,0), w_2 = (0,1) and there exists a market for contingent commodities (x_1, x_2) (consumption in state 1 and 2, respectively) which opens at "t=0" (before state of nature revealed). Now assume state 1 occurs and the consumers can "retrade". Then consumer 1 would sure not fulfill his duties from the contract signed in "period 0". He would just consume his endowment in state 1.

Suggestions?