"The police officer will arrest you when you will come to the party tonight."
Translation:השוטר יעצור אותךָ כשתגיע הערב למסיבה.
In English, a police office is the same thing as a policeman, just gender-neutral.
The English here would be much better without the second "will." It sounds very "translated" as it is.
And in fact I already changed it, so you're commenting on a sentence which no longer exists, technically. ;)
I'm still seeing it with the 2nd "will" so maybe there are 2 versions accepted but the less natural one is coming up as the main solution.