Reject duplicate arrays (#455)

Fixes #454

Author: alexander-buerger-met-no

parse.go

@@ -644,6 +644,11 @@ func (p *parser) setValue(key string, value any) {
 		// Note that since it has already been defined (as a hash), we don't
 		// want to overwrite it. So our business is done.
 		if p.isArray(keyContext) {
+			if !p.isImplicit(keyContext) {
+				if _, ok := hash[key]; ok {
+					p.panicf("Key '%s' has already been defined.", keyContext)
+				}
+			}
 			p.removeImplicit(keyContext)
 			hash[key] = value
 			return

toml_test.go

@@ -315,8 +315,6 @@ func runTomlTest(t *testing.T, includeNext bool, wantFail ...string) {
 			Parser:   parser{},
 			RunTests: runTests,
 			SkipTests: []string{
-				"invalid/key/duplicate-keys-08",
-
 				// Will be fixed in Go 1.23: https://github.com/BurntSushi/toml/issues/407
 				"invalid/datetime/offset-overflow-hour",
 				"invalid/datetime/offset-overflow-minute",