“It is what it is”

Obviously. This is known as a tautophrase: a circular (reinforcing) definition that says the same thing twice.
This phrase pretends to stoic wisdom through acceptance: "it is" beyond control therefore we should move along instead of continuing to consider "it."

The most annoying (and detrimental) aspect is the evasion of communal reflection, and subsequent deferral of responsibility, by claiming fixity through redundancy.

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