Fix crash for double variable declaration in the same scope.

test/libsolidity/syntaxTests/scoping/double_variable_declaration_same_and_disjoint_scope.sol

@@ -0,0 +1,10 @@
+contract test {
+    function f() pure public {
+        uint x;
+        { uint x; }
+        uint x;
+    }
+}
+// ----
+// Warning: (73-79): This declaration shadows an existing declaration.
+// DeclarationError: (91-97): Identifier already declared.

test/libsolidity/syntaxTests/scoping/double_variable_declaration_same_scope.sol

@@ -0,0 +1,8 @@
+contract test {
+    function f() pure public {
+        uint x;
+        uint x;
+    }
+}
+// ----
+// DeclarationError: (71-77): Identifier already declared.

test/libsolidity/syntaxTests/scoping/poly_variable_declaration_same_scope.sol

@@ -0,0 +1,16 @@
+contract test {
+    function f() pure public {
+        uint x;
+        uint x;
+        uint x;
+        uint x;
+        uint x;
+        uint x;
+    }
+}
+// ----
+// DeclarationError: (71-77): Identifier already declared.
+// DeclarationError: (87-93): Identifier already declared.
+// DeclarationError: (103-109): Identifier already declared.
+// DeclarationError: (119-125): Identifier already declared.
+// DeclarationError: (135-141): Identifier already declared.