You would only pay sales tax at the time of sale (which is in Japan). So you would not pay sales tax again.
The CBP uses the Harmonized Tariff Schedule to determine duty rates:
The Customs Duty Rate is a percentage. This percentage is determined
by the total purchased value of the article(s) paid at a foreign
country and not based on factors such as quality, size, or weight. The
Harmonized Tariff System (HTS) provides duty rates for virtually every
existing item. CBP uses the Harmonized Tariff Schedule of the United
States Annotated (HTSUS), which is a reference manual that the
provides the applicable tariff rates and statistical categories for
all merchandise imported into the U.S.
You can search this database online and searching for computer or laptop does not return results that would indicate that there is duty to be paid for these items.
Anecdotal evidence (from many travels) indicates that computer laptops (especially if they are used like yours) are exempt.
Some of my US colleagues also purchase goods abroad and bring them to the US without issue - as they are cheaper here.
New ones, or multiple quantities of laptops do raise suspicions. My friend who was traveling with his family had 8 ipads, 3 laptops, 6 iphones and 2 android phones.
Other than a chuckle and questions as to what he was doing with so many things (till my friend pointed to this kids and wife), he did not have to pay any duties during his trip.