I've heard this phrase a couple times, one of them being
A number is a number is a number.
Despite some searching, I'm still unsure what this phrase means.
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The construction may be understood as an emphasis, a defining emphasis, an emphasis that stops further discussion of "blank".
"A number is a number" is a truism that could be interpreted to mean that a number is a number (agreed as an obvious superfluous statement, but there may be possible nuances and differences that depend on circumstance or interpretation).
For example "A number is a number" is unquestionably true, but most mathematicians immediately think "Yes, but what sort of number? real, integer, rational, irrational, imaginary ... and so forth).
To say "a number is a number is a number" is to say that there is no need or purpose in considering any of these questions. A number is a number, and that is the end of the matter; think no further; any more speculation or qualification, as to what sort of number or any other issues, is not required or wanted.