🐛 Fixed duplication error when importing posts without slugs

refs #8717

-  The posts without slugs should not be taken into account when detecting duplicates as slug field is not required when importing.
- Ideal solution would require generating slug before duplicate detection phase. This would cause duplicate detection to take 'title' into account which didn't happen before.

core/server/data/importer/importers/data/posts.js

@@ -200,7 +200,7 @@ class PostsImporter extends BaseImporter {
         // For any further future duplication detection, see https://github.com/TryGhost/Ghost/issues/8717.
         let slugs = [];
         this.dataToImport = _.filter(this.dataToImport, (post) => {
-            if (slugs.indexOf(post.slug) !== -1) {
+            if (!!post.slug && slugs.indexOf(post.slug) !== -1) {
                 this.problems.push({
                     message: 'Entry was not imported and ignored. Detected duplicated entry.',
                     help: this.modelName,

core/test/regression/importer/importer_spec.js

@@ -252,6 +252,31 @@ describe('Integration: Importer', function () {
                 });
         });
 
+        it('does not treat posts without slug as duplicate', function () {
+            let exportData = exportedLatestBody().db[0];
+
+            exportData.data.posts[0] = {
+                title: "duplicate title"
+            };
+
+            exportData.data.posts[1] = {
+                title: "duplicate title"
+            };
+
+            return dataImporter.doImport(exportData, importOptions)
+                .then(function (importResult) {
+                    should.exist(importResult.data.posts);
+                    importResult.data.posts.length.should.equal(2);
+                    importResult.problems.length.should.eql(0);
+
+                    importResult.data.posts[0].title.should.equal('duplicate title');
+                    importResult.data.posts[1].title.should.equal('duplicate title');
+
+                    importResult.data.posts[0].slug.should.equal('duplicate-title');
+                    importResult.data.posts[1].slug.should.equal('duplicate-title-2');
+                });
+        });
+
         it('can import user with missing allowed fields', function () {
             let exportData = exportedLatestBody().db[0];