FAYETTEVILLE, Ark. (KNWA/KFTA/NEXSTAR) — Have a few $1 bills lying around? Before you spend them, you may want to check if they’re worth even more than a buck.

Earlier this week, Wealthy Nickel, a finance blog, reminded currency collectors of an error the Bureau of Engraving and Printing made in the last decade.

As you likely know, every paper note the U.S. prints — from the lowly $1 bill to the $100 bill — is printed with a serial number. Like your Social Security number, there shouldn’t be any duplicates. But, according to reports from Numismatic News and paper money grading service PMG, there are more than 6 million $1 bills that share a serial number with another $1 bill.

According to both outlets, 10 years ago, the Bureau of Engraving and Printing instructed its printing facilities in Washington, D.C., and Fort Worth, Texas, to print millions of $1 bills. Unfortunately, those bills sported matching numbers.

To be clear, this misprint doesn’t make the money unusable. Serial numbers serve as a way to denote which Federal Reserve Bank issued the bill, the series in which it was printed, and which of the millions printed it is.

In fact, the duplicates seemingly went unnoticed until two collectors, Ed Zegers of Maryland and Karol Winograd of Florida, noticed the inconsistency. They decided to launch an effort to find the error pairs in 2017, according to Numismatic News. By the end of 2020, the pair had found 10 pairs.

Their database of duplicate notes has since been transferred to Project 2013B, which recently reported its 16th match of $1 bills. The project boasts more than 47,000 $1 bills sporting serial numbers from the error batches, the majority of which are partner-less.

As Wealthy Nickel explains, there are three ways to determine if your $1 bill is part of this collection:

• “Series 2013” should appear to the right of President George Washington’s image
• To the left of Washington, there should be a seal with a “B” in it
• Below that, the serial number should end with a star, and fall between B00000001 – B00250000 or B03200001-B09600000

It’s difficult to say how much these $1 bills could be worth, but they’re seemingly better in pairs. A set auctioned by Stack’s Bowers sold for $7,200 in 2021. PMG, which graded both bills, ruled the bill from Fort Worth as 66 Gem Uncirculated EPQ (the scale goes up to 70) while its Washington twin was a PMG 35 Choice Very Fine. Another set was reportedly sold on eBay for $25,000.

As with any currency you think may be worth more than its printed value — from wheat pennies and buffalo nickels to $2 bills and error notes — you should speak with a verified expert for evaluation.

“Odd items are always set aside, but that doesn’t make them rare or terribly valuable,” Dustin Johnson, vice president of Numismatics at Heritage Auctions, told Nexstar last year.