Some internet browsers will remember commonly used addresses and seek to auto-populate the address for you.
This why duplications can occur. For example:
You can easily change this.
1. Go to your Account
2. Select the blue 'Edit account details,' button on the right hand side.
3. Select 'Add a new address.'
4. Add just the House name/number and Postcode. Then click, 'Find Address' which will populate the rest of the address:
Just make sure your internet browser does not unhelpfully add anything extra to the completed address before saving. For instance, it might insert '5 Queen Street' into Address line 1 rather than just leaving it as 'Queen Street' and so the 5 will be duplicated giving 5 5 Queen Street!
Note: If your internet browser populates any fields, please ensure that there is no duplicate information shown.