U4-4629 - Detecting existing macros

By setting the ID of the macro the repository layer will update the existing instance rather than trying to create a new one, which would result in a duplicate alias and that violates a table index.

src/Umbraco.Core/Services/PackagingService.cs

@@ -1196,6 +1196,10 @@ namespace Umbraco.Core.Services
 
             foreach (var macro in macros)
             {
+                var existing = _macroService.GetByAlias(macro.Alias);
+                if (existing != null)
+                    macro.Id = existing.Id;
+
                 _macroService.Save(macro, userId);
             }
 
@@ -1820,4 +1824,4 @@ namespace Umbraco.Core.Services
         public static event TypedEventHandler<IPackagingService, ExportEventArgs<ITemplate>> ExportedTemplate;
         #endregion
     }
-}
+}