Fix model overwrite error when plugin is initialized multiple times

- Filter out existing collections with same slugs before adding plugin collections
- Prevents 'Cannot overwrite model once compiled' errors in Next.js apps
- Fixes issue during hot reload and multiple getPayload() calls
- Bump version to 0.1.19

src/plugin.ts

@@ -74,10 +74,15 @@ export const mailingPlugin = (pluginConfig: MailingPluginConfig) => (config: Con
     }),
   } satisfies CollectionConfig
 
+  // Filter out any existing collections with the same slugs to prevent duplicates
+  const existingCollections = (config.collections || []).filter(
+    (collection) => collection.slug !== templatesSlug && collection.slug !== emailsSlug
+  )
+
   return {
     ...config,
     collections: [
-      ...(config.collections || []),
+      ...existingCollections,
       templatesCollection,
       emailsCollection,
     ],