diff --git a/packages/combo-box/src/vaadin-combo-box-mixin.js b/packages/combo-box/src/vaadin-combo-box-mixin.js
index 9bd16b691a4..bcddb3eddf3 100644
--- a/packages/combo-box/src/vaadin-combo-box-mixin.js
+++ b/packages/combo-box/src/vaadin-combo-box-mixin.js
@@ -698,6 +698,7 @@ export const ComboBoxMixin = (subclass) =>
         this.selectedItem = null;
 
         if (this.allowCustomValue) {
+          delete this._lastCustomValue;
           this.value = '';
         }
       } else {
@@ -714,17 +715,27 @@ export const ComboBoxMixin = (subclass) =>
           // to prevent a repetitive input value being saved after pressing ESC and Tab.
           !itemMatchingByLabel
         ) {
+          const customValue = this._inputElementValue;
+
+          // User's logic in `custom-value-set` event listener might cause input to blur,
+          // which will result in attempting to commit the same custom value once again.
+          if (this._lastCustomValue === customValue) {
+            return;
+          }
+
+          // Store reference to the last custom value for checking it
+          this._lastCustomValue = customValue;
+
           // An item matching by label was not found, but custom values are allowed.
           // Dispatch a custom-value-set event with the input value.
           const e = new CustomEvent('custom-value-set', {
-            detail: this._inputElementValue,
+            detail: customValue,
             composed: true,
             cancelable: true,
             bubbles: true
           });
           this.dispatchEvent(e);
           if (!e.defaultPrevented) {
-            const customValue = this._inputElementValue;
             this._selectItemForValue(customValue);
             this.value = customValue;
           }
diff --git a/packages/combo-box/test/basic.test.js b/packages/combo-box/test/basic.test.js
index 900f27043c1..ce2f611c6cf 100644
--- a/packages/combo-box/test/basic.test.js
+++ b/packages/combo-box/test/basic.test.js
@@ -271,6 +271,59 @@ describe('Properties', () => {
 
         expect(spy.calledOnce).to.be.true;
       });
+
+      it('should not fire twice when the custom value set listener causes blur', () => {
+        const spy = sinon.spy();
+        comboBox.addEventListener('custom-value-set', spy);
+
+        // Emulate opening the overlay that causes blur
+        comboBox.addEventListener('custom-value-set', () => {
+          comboBox.blur();
+        });
+
+        comboBox.open();
+        input.value = 'foo';
+        input.dispatchEvent(new CustomEvent('input'));
+        comboBox.close();
+
+        expect(spy.calledOnce).to.be.true;
+      });
+
+      it('should fire twice when another custom value is committed by the user', () => {
+        const spy = sinon.spy();
+        comboBox.addEventListener('custom-value-set', spy);
+
+        comboBox.open();
+        input.value = 'foo';
+        input.dispatchEvent(new CustomEvent('input'));
+        comboBox.close();
+
+        input.value = 'bar';
+        input.dispatchEvent(new CustomEvent('input'));
+        focusout(input);
+
+        expect(spy.calledTwice).to.be.true;
+      });
+
+      it('should fire when setting the same custom value after clearing', () => {
+        const spy = sinon.spy();
+        comboBox.addEventListener('custom-value-set', spy);
+
+        input.value = 'foo';
+        input.dispatchEvent(new CustomEvent('input'));
+        focusout(input);
+
+        input.value = '';
+        input.dispatchEvent(new CustomEvent('input'));
+        focusout(input);
+
+        spy.resetHistory();
+        input.value = 'foo';
+        input.dispatchEvent(new CustomEvent('input'));
+        focusout(input);
+
+        expect(spy.calledOnce).to.be.true;
+      });
     });
   });