checker: check struct field name duplicate

vlib/v/checker/checker.v

@@ -203,7 +203,12 @@ pub fn (mut c Checker) struct_decl(decl ast.StructDecl) {
 		}
 		c.error('struct name must begin with capital letter', pos)
 	}
-	for field in decl.fields {
+	for i, field in decl.fields {
+		for j in 0..i {
+			if field.name == decl.fields[j].name {
+				c.error('field name `$field.name` duplicate', field.pos)
+			}
+		}
 		sym := c.table.get_type_symbol(field.typ)
 		if sym.kind == .placeholder && !decl.is_c && !sym.name.starts_with('C.') {
 			c.error('unknown type `$sym.name`', field.pos)

vlib/v/checker/tests/struct_field_name_duplicate_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_field_name_duplicate_err.v:3:4: error: field name `a` duplicate
+    1| struct A {
+    2|     a int
+    3|     a string
+             ~~~~~~
+    4| }
+    5| fn main(){}

vlib/v/checker/tests/struct_field_name_duplicate_err.v

@@ -0,0 +1,5 @@
+struct A {
+	a int
+	a string
+}
+fn main(){}

vlib/v/checker/tests/struct_field_name_duplicate_err.vv

@@ -0,0 +1,5 @@
+struct A {
+	a int
+	a string
+}
+fn main(){}