Fail on duplicate keys in a mapping #328

Job	Run time
Linux (5.4)	38s
Windows (swift-5.6.3-release, 5.6.3-RELEASE)	1m 49s
Linux (5.5)	45s
Windows (swift-5.7.2-release, 5.7.2-RELEASE)	1m 50s
Linux (5.6)	42s
Windows (development, DEVELOPMENT-SNAPSHOT-2021-11-20-a)	1m 49s
Linux (5.7)	36s
Linux (5.8)	41s
Linux (5.9)	43s
Xcode (12.5.1)	1m 16s
Linux (5.10)	39s
Xcode (13.0)	1m 32s
Xcode (13.1)	1m 25s
Xcode (13.2.1)	1m 26s
Xcode_Monterey (13.3.1)	1m 37s
Xcode_Monterey (13.4)	1m 30s
Xcode_Monterey (14.0.1)	1m 8s
Xcode_Ventura (14.3)	55s
Xcode_Monterey (14.1)	1m 32s
Xcode_Monterey (14.2)	2m 21s
Xcode_Ventura (15.0)	1m 22s
	26m 16s

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