UHB?
Restating: erase each pair of duplicate letters.
For simplicity, start at the left. Scan right to find a match.
THE UNITED STATES ONE HUNDRED DOLLAR BILL
_HE UNI_ED STATES ONE HUNDRED DOLLAR BILL
Repeat, starting again at the left.
__E UNI_ED STATES ONE__UNDRED DOLLAR BILL
The underscores are merely placeholders for spacing.
Any letters remaining?