diff --git a/src/plots/cartesian/ordered_categories.js b/src/plots/cartesian/ordered_categories.js
index 546578ecb15..2666bb71170 100644
--- a/src/plots/cartesian/ordered_categories.js
+++ b/src/plots/cartesian/ordered_categories.js
@@ -41,7 +41,7 @@ function flattenUniqueSort(axisLetter, sortFunction, data) {
             insertionIndex = bisector(categoryArray, category);
 
             // skip loop on already encountered values
-            if(insertionIndex < categoryArray.length - 1 && categoryArray[insertionIndex] === category) continue;
+            if(insertionIndex < categoryArray.length && categoryArray[insertionIndex] === category) continue;
 
             // insert value
             categoryArray.splice(insertionIndex, 0, category);
diff --git a/test/jasmine/tests/calcdata_test.js b/test/jasmine/tests/calcdata_test.js
index fb9c3049994..1a6b1c01d4b 100644
--- a/test/jasmine/tests/calcdata_test.js
+++ b/test/jasmine/tests/calcdata_test.js
@@ -137,6 +137,18 @@ describe('calculated data and points', function() {
                 expect(gd.calcdata[0][3]).toEqual(jasmine.objectContaining({x: 0, y: 13}));
                 expect(gd.calcdata[0][4]).toEqual(jasmine.objectContaining({x: 2, y: 14}));
             });
+
+            it('should combine duplicate categories', function() {
+                Plotly.plot(gd, [{x: [ '1', '1'], y: [10, 20]}], { xaxis: {
+                    type: 'category',
+                    categoryorder: 'category ascending'
+                }});
+
+                expect(gd.calcdata[0][0]).toEqual(jasmine.objectContaining({x: 0, y: 10}));
+                expect(gd.calcdata[0][1]).toEqual(jasmine.objectContaining({x: 0, y: 20}));
+
+                expect(gd._fullLayout.xaxis._categories).toEqual(['1']);
+            });
         });
 
         describe('explicit category ordering', function() {