Fixed duplicated inline diff discussions

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55388

app/assets/javascripts/diffs/store/mutations.js

@@ -138,7 +138,7 @@ export default {
         if (file.highlighted_diff_lines) {
           file.highlighted_diff_lines = file.highlighted_diff_lines.map(line =>
-            mapDiscussions(line),
+            lineCheck(line) ? mapDiscussions(line) : line,
           );
         }

spec/javascripts/diffs/store/mutations_spec.js

@@ -358,6 +358,71 @@ describe('DiffsStoreMutations', () => {
       expect(state.diffFiles[0].highlighted_diff_lines[0].discussions[0].resolved).toBe(true);
     });
+    it('should not duplicate inline diff discussions', () => {
+      const diffPosition = {
+        base_sha: 'ed13df29948c41ba367caa757ab3ec4892509910',
+        head_sha: 'b921914f9a834ac47e6fd9420f78db0f83559130',
+        new_line: null,
+        new_path: '500-lines-4.txt',
+        old_line: 5,
+        old_path: '500-lines-4.txt',
+        start_sha: 'ed13df29948c41ba367caa757ab3ec4892509910',
+      };
+
+      const state = {
+        latestDiff: true,
+        diffFiles: [
+          {
+            file_hash: 'ABC',
+            highlighted_diff_lines: [
+              {
+                line_code: 'ABC_1',
+                discussions: [
+                  {
+                    id: 1,
+                    line_code: 'ABC_1',
+                    diff_discussion: true,
+                    resolvable: true,
+                    original_position: diffPosition,
+                    position: diffPosition,
+                    diff_file: {
+                      file_hash: 'ABC',
+                    },
+                  },
+                ],
+              },
+              {
+                line_code: 'ABC_2',
+                discussions: [],
+              },
+            ],
+          },
+        ],
+      };
+      const discussion = {
+        id: 2,
+        line_code: 'ABC_2',
+        diff_discussion: true,
+        resolvable: true,
+        original_position: diffPosition,
+        position: diffPosition,
+        diff_file: {
+          file_hash: state.diffFiles[0].file_hash,
+        },
+      };
+
+      const diffPositionByLineCode = {
+        ABC_2: diffPosition,
+      };
+
+      mutations[types.SET_LINE_DISCUSSIONS_FOR_FILE](state, {
+        discussion,
+        diffPositionByLineCode,
+      });
+
+      expect(state.diffFiles[0].highlighted_diff_lines[0].discussions.length).toBe(1);
+    });
+
     it('should add legacy discussions to the given line', () => {
       const diffPosition = {
         base_sha: 'ed13df29948c41ba367caa757ab3ec4892509910',