The party that made the overpayment would have the right to sue you for "unjust enrichment" or "breach of contract" (since the terms of service no doubt provide or strongly imply that you are entitled to only one payment per sale), if you didn't voluntarily return the overpayment following a demand to do so, even though you received it through no fault of your own. Most of the core cases involve clerical errors in the bank account number used for a bank deposit.
In general, there is a right to recover an accidental transfer of property to another, even in the absence of a clearly applicable contractual arrangement. As another example, if you were accidentally given a valuable coat at a coat check by accident and didn't notice it until later, the true owner would have a right to have it returned.
The FTC regulation applies (as demonstrated by the link cited in the question) to intentional unsolicited deliveries of merchandise (which would always be tangible personal property by definition) to you through the mail. It does not apply to transfers of money, or to the accidental mis-delivery of property to the wrong person or the wrong address.
The law in question is as follows:
39 U.S. Code § 3009 - Mailing of unordered merchandise
(a) Except for (1) free samples clearly and conspicuously marked as
such, and (2) merchandise mailed by a charitable organization
soliciting contributions, the mailing of unordered merchandise or of
communications prohibited by subsection (c) of this section
constitutes an unfair method of competition and an unfair trade
practice in violation of section 45(a)(1) of title 15.
(b) Any merchandise mailed in violation of subsection (a) of this
section, or within the exceptions contained therein, may be treated as
a gift by the recipient, who shall have the right to retain, use,
discard, or dispose of it in any manner he sees fit without any
obligation whatsoever to the sender. All such merchandise shall have
attached to it a clear and conspicuous statement informing the
recipient that he may treat the merchandise as a gift to him and has
the right to retain, use, discard, or dispose of it in any manner he
sees fit without any obligation whatsoever to the sender.
(c) No mailer of any merchandise mailed in violation of subsection (a)
of this section, or within the exceptions contained therein, shall
mail to any recipient of such merchandise a bill for such merchandise
or any dunning communications.
(d) For the purposes of this section, “unordered merchandise” means
merchandise mailed without the prior expressed request or consent of