Forbid duplicate namespace/static class members #4674

Job	Run time
Check	6s
Compiler (ubuntu, node current)	2m 7s
Compiler (ubuntu, node lts/*)	2m 2s
Coverage	3m 36s
Features	20s
Compiler (macos, node current)	4m 51s
Bootstrap (debug)	6m 14s
Compiler (macos, node lts/*)	3m 9s
Bootstrap (release)	6m 48s
Loader	19s
Compiler (windows, node current)	2m 25s
Runtimes	34s
Compiler (windows, node lts/*)	2m 26s
	34m 57s

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