Use error variant for field duplicates

instant-xml-macros/src/de.rs

@@ -259,7 +259,7 @@ impl Deserializer {
             tokens.match_.extend(quote!(
                 __Elements::#enum_name => {
                     if #enum_name.is_some() {
-                        panic!("duplicated value");
+                        return Err(Error::DuplicateValue);
                     }
 
                     let mut nested = deserializer.nested(data);
@@ -270,7 +270,7 @@ impl Deserializer {
             tokens.match_.extend(quote!(
                 __Attributes::#enum_name => {
                     if #enum_name.is_some() {
-                        panic!("duplicated value");
+                        return Err(Error::DuplicateValue);
                     }
 
                     let mut nested = deserializer.for_attr(attr);

instant-xml/src/lib.rs

@@ -90,4 +90,6 @@ pub enum Error {
     ExpectedScalar,
     #[error("wrong namespace")]
     WrongNamespace,
+    #[error("duplicate value")]
+    DuplicateValue,
 }