If you started from the example of myfaces sandbox and eventually got it working, implemented your ownbacking bean but receive duplicate suggestions. In the logs you see that the backing bean is also called twice... The problem may lay in using an example config file with a lot of sandbox components registered there (especially if you took it from SVN), such as:
<component>
<component-type>org.apache.myfaces.InputSuggestAjax</component-type>
<component-class>org.apache.myfaces.custom.inputsuggestajax.InputSuggestAjax</component-class>
</component>
To solve the problem make the
<faces-config><faces-config/> empty and start from scratch (i.e. register there only your own backing beans and remove all those components!).
