Fix duplicate symbol value fields when importing Eagle schematic.

Don't use the value field when adding Eagle part attributes to a symbol.
This issue was cause by using a mandatory symbol field ID instead of
using the next available field ID when adding new fields to as symbol.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/13468

eeschema/sch_plugins/eagle/sch_eagle_plugin.cpp

@@ -1717,10 +1717,19 @@ void SCH_EAGLE_PLUGIN::loadInstance( wxXmlNode* aInstanceNode )
 
     for( const auto& a : epart->attribute )
     {
-        SCH_FIELD* field = symbol->AddField( *symbol->GetField( VALUE_FIELD ) );
-        field->SetName( a.first );
-        field->SetText( a.second );
-        field->SetVisible( false );
+        VECTOR2I newFieldPosition( 0, 0 );
+        SCH_FIELD* lastField = symbol->GetFieldById( symbol->GetFieldCount() - 1 );
+
+        if( lastField )
+            newFieldPosition = lastField->GetPosition();
+
+        SCH_FIELD newField( newFieldPosition, symbol->GetFieldCount(), symbol.get() );
+
+        newField.SetName( a.first );
+        newField.SetText( a.second );
+        newField.SetVisible( false );
+
+        symbol->AddField( newField );
     }
 
     for( const auto& a : epart->variant )