Return an error if there are two enums with the same name #1510

Job	Run time
Check	2m 18s
build_nightly	2m 59s
build_examples	6m 43s
build_benchmarks	1m 51s
Verify that web-sys is compiled correctly	1m 14s
Clippy	9m 59s
dist_linux_aarch64_gnu	4m 57s
dist_linux_x86_64_musl	4m 38s
dist_macos_aarch64	3m 6s
dist_macos_x86_64	5m 25s
dist_windows	4m 52s
doc_api	1m 30s
doc_book	22s
Rustfmt	12s
Build and test the deno example	1m 15s
Run js-sys crate tests	3m 21s
Run native tests	3m 17s
Run wasm-bindgen crate tests with multithreading enabled	2m 6s
Test TypeScript output of wasm-bindgen	2m 8s
Run UI compile-fail tests	5m 28s
Run wasm-bindgen crate tests (unix)	11m 29s
Run web-sys crate tests	8m 51s
Run wasm-bindgen-webidl crate tests	4m 48s
test_examples	3m 31s
deploy	1m 48s
	1h 38m 8s

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