Depends on what the definition of "accepted" is.
By "accepted" do you mean that they received the contract? Or by "accepted" do you mean that you received a signed, executed contract?
If you received a signed, executed contract, then no they cannot "accept" another offer in less than an hour. If you mean they received the contract, then the sellers can "accept" any number of offers. Depending on the circumstances, it is not unusual to have a "Multiple Offer Situation," meaning that they have accepted several offers and then they turn around and request from all of the offers their "Highest and Best" offer by a certain deadline.
I hope this answers your question. If there are other details here that you did not mention, feel free to follow up and I'm sure you will get some help with your question.
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Brian Rayl, REALTORÂ®, e-PRO, SFR
Keller Williams Elite