parser: add checks for duplicate operator overload

vlib/v/checker/tests/method_op_err.out

@@ -1,7 +1,7 @@
 vlib/v/checker/tests/method_op_err.vv:11:1: error: operator methods should have exactly 1 argument
     9 | }
    10 | 
-   11 | fn (u User) + () {
+   11 | fn (u User) % () {
       | ~~~~~~~~~~~~~~~~
    12 | }
    13 |
@@ -68,7 +68,7 @@ vlib/v/checker/tests/method_op_err.vv:38:5: error: operator %= not defined on le
       |     ^
    39 |     u += User{2, 3}
    40 | }
-vlib/v/checker/tests/method_op_err.vv:38:7: error: undefined operation `User` % `User`
+vlib/v/checker/tests/method_op_err.vv:38:7: error: operator `%` must return `User` to be used as an assignment operator
    36 |    _ = u
    37 |     u += 12
    38 |     u %= User{1, 3}

vlib/v/checker/tests/method_op_err.vv

@@ -8,7 +8,7 @@ struct Foo {
     b int
 }
 
-fn (u User) + () {
+fn (u User) % () {
 }
 
 fn (u User) - (f Foo) User {

vlib/v/parser/fn.v

@@ -362,6 +362,9 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			p.error_with_pos('cannot use operator overloading with normal functions',
 				p.tok.pos())
 		}
+		if type_sym.has_method(name) {
+			p.error_with_pos('cannot duplicate operator overload `${name}`', p.tok.pos())
+		}
 		p.next()
 	} else if p.tok.kind in [.ne, .gt, .ge, .le] && p.peek_tok.kind == .lpar {
 		p.error_with_pos('cannot overload `!=`, `>`, `<=` and `>=` as they are auto generated from `==` and`<`',

vlib/v/parser/tests/duplicate_operator_overload_err.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/duplicate_operator_overload_err.vv:3:12: error: cannot duplicate operator overload `*`
+    1 | struct Foo{}
+    2 | fn (f Foo) * (f2 Foo) Foo{}
+    3 | fn (f Foo) * (f2 Foo) Foo{}
+      |            ^

vlib/v/parser/tests/duplicate_operator_overload_err.vv

@@ -0,0 +1,3 @@
+struct Foo{}
+fn (f Foo) * (f2 Foo) Foo{}
+fn (f Foo) * (f2 Foo) Foo{}