1954 $20

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is this an error?

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This is a tough one to determine in the scan.
The note appears to have been wet at some point bleeding the serial number through to the back, which commonly happenes on the 54 series notes when wet

The transfer of the front of the note design to the back is known as "offset printing".
It is very hard to determine if this happened at the printing company (legitimate error), or if it happened while the note was in storage, a wet transfer, heat press etc. (a made-up error).

Without seeing the note, that is the best I can do.