Do You Know How to Read Rishonim?

Here’s a riddle. The Rambam (Avoda Zara 2:9) writes:

מגדף שחזר בו בתוך כדי דיבור אינו כלום אלא כיון שגידף בעדים נסקל

What could he mean? What does the first half of the sentence have to do with the other half?

Explain all the words. Then read our answer here.