I seriously doubt it. The only case where something like this is even remotely possible is if the check was mailed using US postal service, a different set of rules apply. Using normal principles, if they just give somebody too much money, they should be able to get it back.
So it could be, if Payroll sends checks to folks’ homes through the USPS?
This thread has wandered so I guess it’s OK to relate this. From '81 through '90 I had a contractural relationship w/ a large private company. Several months before I terminated the contract they paid me twice for the same job. The amount was $3200.00 and change. I called the accounting department and they told me I was mistaken (they were famous for this, denying errors). I tried again about a week or so later, w/ the same result. I made a note of the two calls, including the date, time and name of the person I talked to.
The contract was worth around 50-60K a year to me, so this wasn’t a lot in comparison.
Several weeks after I terminated, I rec’d. a letter stating that I owed the money. I replied and gave them the information about the two phone calls. I fully expected to hear from them again, or from a collection agency. Not so, I never heard another word. I also had several hundred dollars worth of equpment that belonged to the company. They were supposed to send a truck to pick it up. I contacted them several times to remind them and was always told they would contact me, they never did and I finally stopped calling.
They did still owe me around 5-6 hundred at the time of termination, which I never received, but I figured I’d let it slide. 
I still feel it is unlikely, since most of the rules about keeping things that come through the mail have to do with unsolicited mail, not things you are expecting.
Even if they can’t force one to return the money can’t they just terminate one? Of course a free million dollars is probally worth getting fired.
It is not illegal to cash any legitimate instrument. Whether you get to keep the proceeds is a different matter. For any significant amount, the payor is going to take you to court. The remaining proceeds will be confiscated and put in escrow until “final determination.” Then you will have to prove to a reasonable degree that you believed you were entitled to the money. In general, if it’s decided that you could not have reasonably believed that you were entitled to the money, the case will be decided against you and you will have to repay the funds as well as pay your own legal fees.
You are under no obligation to voluntarily pay the money back. If the company does not come after you legally, either by suing you or having you sign a…(I can’t remember the legal name of the document)… <document> stating that you received the money; know you aren’t entitled to it; agree to pay it back; and if you don’t they can get an immediate judgment against you for the balance owed.
Banks and companies often secure such <documents> even when the funds are criminally obtained because it would cost too much to prosecute the case in civil court. (A criminal conviction, in general, does not redress financial loss).
So, while your company may not sue you for the return of a $1,000 payroll overpayment (that you reasonably believed was a bonus) because it would cost them more in legal fees than the value of the suit, you can bet they’ll go after you for $100,000. In addition, if you are recalcitrant about agreeing to repay the mispaid funds, you probably will no longer be working for them even though the proximate cause of your dismissal will have nothing to do with the erroneous payment.
For the guy who received a duplicate payment of $3,200, the first sentence in the above paragraph applies. In addition, the company repeatedly validated the payment. In fact, they probably still owe you the $600+.