Optimize `set_non_blocking`: Avoid dup mono #1089

Job	Run time
Test aarch64-apple-tvos	1m 59s
Test CUDA support	30s
Clippy	14s
MSRV (ubuntu-latest)	21s
Rustfmt	4s
MSRV (windows-latest)	41s
Test (stable)	23s
Test (beta)	33s
Test (nightly)	40s
Test (linux32)	50s
Test (macos)	2m 7s
Test (aarch64-macos)	46s
Test (aarch64-ios)	1m 17s
Test (win32)	1m 18s
Test (win64)	2m 30s
Test (mingw32)	1m 38s
Test (mingw64)	1m 21s
Test (windows-2019)	2m 18s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	57s
	20m 27s

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