Error: Mobile number already exists! while adding contacts to Cute SMS account

This simply means that the mobile number you're trying to add is already added to your Cute SMS account. You can easily check the details and existence of such mobile number at any time by following the steps below.
1. Login to your Cute SMS Account.
2. In the Contact section, click Manage Contact.
3. Select the Contact Group - All from the drop down list of Group Name in the left pane of the window
4. Enter the mobile number that you want to add in Mobile No. field and Click on Search button next to it. This will display the contact details.
5. On the Manage Contact segment, select the contact that you want to add in another Contact Group and then click Edit option.
6. Edit Contacts window appears. Check preferred Group Names wherever you want to keep the contact added.
7. Click Save to save changes.
8. You will see a confirmation message as follows: Contact modified successfully.