Epenthesis
In Dutch, when the suffix -er (which has several meanings) is placed after a word which already ends in -r, another  is put in the middle. For example, the comparative form (the form people use to compare things) of the adjective zoet ("sweet") is zoeter, but the comparative of zuur ("sour") is zuurder and not the expected *. The same happens with the agent (someone who does something) noun form of verkopen ("to sell") which is verkoper ("salesperson"), but the agent noun of uitvoeren ("to perform") is uitvoerder ("performer").