E.g. s149(1)(a), Equality Act 2010 says:
A public authority must, in the exercise of its functions, have due regard to the need to—
(a)eliminate discrimination, harassment, victimisation and any other conduct that is prohibited by or under this Act;
Why does it specify both words, “by” and “under” when they both apparently mean the same thing?
"By or under" is redundant. It is a legal doublet.
The Australian Capital Territory Parliamentary Counsel's Office's Words and Phrases Guide (at B-10 and at xiii) says to "never use" "by and/or under" and to instead simply use "under."
A drafting point at p. 30 of a "Compilation of comments and proposals by Governments and by intergovernmental and non-governmental organizations on the draft articles for a convention on arrest of ships":
The words “by or under” (the authority) seem to be redundant. It is suggested that the words “by or” be deleted so that the text would read: “ ... only under the authority ...”