No duplicate property accessor should be allowed

compiler/testData/psi/DuplicateAccessor.kt

@@ -0,0 +1,3 @@
+val a : Int
+    get() = 0
+    get() = 0

compiler/testData/psi/DuplicateAccessor.txt

@@ -0,0 +1,39 @@
+JetFile: DuplicateAccessor.kt
+  PACKAGE_DIRECTIVE
+    <empty list>
+  IMPORT_LIST
+    <empty list>
+  PROPERTY
+    PsiElement(val)('val')
+    PsiWhiteSpace(' ')
+    PsiElement(IDENTIFIER)('a')
+    PsiWhiteSpace(' ')
+    PsiElement(COLON)(':')
+    PsiWhiteSpace(' ')
+    TYPE_REFERENCE
+      USER_TYPE
+        REFERENCE_EXPRESSION
+          PsiElement(IDENTIFIER)('Int')
+    PsiWhiteSpace('\n    ')
+    PROPERTY_ACCESSOR
+      PsiElement(get)('get')
+      PsiElement(LPAR)('(')
+      PsiElement(RPAR)(')')
+      PsiWhiteSpace(' ')
+      PsiElement(EQ)('=')
+      PsiWhiteSpace(' ')
+      INTEGER_CONSTANT
+        PsiElement(INTEGER_LITERAL)('0')
+  PsiWhiteSpace('\n    ')
+  PsiErrorElement:Expecting a top level declaration
+    PsiElement(get)('get')
+  PsiErrorElement:Expecting a top level declaration
+    PsiElement(LPAR)('(')
+  PsiErrorElement:Expecting a top level declaration
+    PsiElement(RPAR)(')')
+  PsiWhiteSpace(' ')
+  PsiErrorElement:Expecting a top level declaration
+    PsiElement(EQ)('=')
+  PsiWhiteSpace(' ')
+  PsiErrorElement:Expecting a top level declaration
+    PsiElement(INTEGER_LITERAL)('0')