different between onetime vs oneoff

English

one +? time

onetime (not comparable)

Alternative form of one-time (“former; past”)
- 2004, The 9-11 Commission, 9/11 Commission Report, Chapter 4:
  A onetime special […] forces officer, he had worked with Albright when she was ambassador to the United Nations and had served on the NSC staff with Clarke.
Alternative form of one-time (“occurring once”)
- 2004, The 9-11 Commission, 9/11 Commission Report, Chapter 6:
  The additional funds included the FBI’s support of the 2002 Winter Olympics in Salt Lake City, Utah (a onetime increase) […]

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