Fix missing checks for duplicate literals in variants with payloads. #6681

Job	Run time
build-rewatch (macos-13, x86_64-apple-darwin, darwin-x64)	18s
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	1m 47s
build-rewatch (macos-14, aarch64-apple-darwin, darwin-arm64)	15s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	3m 57s
build-rewatch (ubuntu-24.04, x86_64-unknown-linux-musl, linux-x64)	14s
build-compiler (macos-13, 5.3.0, true, darwin-x64)	4m 36s
build-rewatch (ubuntu-24.04-arm, aarch64-unknown-linux-musl, linux-arm64)	25s
build-compiler (macos-14, 5.3.0, true, darwin-arm64)	2m 18s
build-compiler (windows-latest, 5.3.0, true, win32-x64)	3m 27s
build-rewatch (windows-latest, x86_64-pc-windows-gnu, win32-x64)	37s
build-compiler (ubuntu-24.04, ocaml-variants.5.2.1+options,ocaml-option-static)	2m 7s
build-compiler (ubuntu-24.04, ocaml-variants.5.0.0+options,ocaml-option-static)	2m 16s
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static)	2m 5s
pkg-pr-new	58s
installationTest (macos-13, darwin-x64)	23s
installationTest (macos-14, darwin-arm64)	14s
installationTest (ubuntu-24.04, linux-x64)	12s
installationTest (ubuntu-24.04-arm, linux-arm64)	21s
installationTest (windows-latest, win32-x64)	54s
publish	0s
	27m 24s

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