First, there is no rule or regulation or even ethical standard that mandates that a seller look at one offer before another. In MN a seller can do pretty much whatever they like with an offer (whether it is a good idea or not is whole seperate issue).
One thing that is different in MN as opposed to some other states is that a seller can only sign and fully execute one offer. Once they have done this, legally and technically (I say this because it does not take reality into complete account), the seller has to work with you first. The reason this isn't reality is that the only way that you can enforce a contract is through suing the seller for specific performance. While every agent will scream "specific performance" at the first sign of trouble, it is rarely pursued because the court battle that ensues is lengthy and expensive. It is almost always better to lick your wounds and go out and find something else.
A couple of corrections that I wanted to bring up. Rob (sorry friend) brings up a very common misconception in real estate and sadly it is a view that is heald by most realtors as well. Time is of the essence does not mean that people must hurry but rather it means that the dates and times in the contract are real and must be adheared to. It is not a unique legal concept to real estate transactions but rather can be applied to all of contract law.
Their is an ethical standard that all Realtors (only members of the national association of realtors) must adhear to as listing agents. Standard of Practice 1-6 states says that: "REALTORSÂ® shall submit offers and counter-offers objectively and as quickly as possible". All this means however is that the listing agent must present your offer to the seller quickly, it says nothing of the sellers response time.
I hope all of that helps. Do let us know if you have any other questions.