我晚上要去健身房锻炼一下才会去聚会。

我晚上要先去健身房锻炼锻炼再来聚会。 I'm going to the gym for an evening workout before I come to the party.

Two major ways in Mandarin to make things less formal: Method 1:We repeat the verb. (锻炼锻炼） Method 2:We add 一下 after the verb. （ 锻炼一下） Either way, we are making it sound casual. But we wouldn't use both of them in a sentence.（* 锻炼锻炼一下）

我晚上要先去健身房锻炼锻炼才来聚会。OR 我晚上要先去健身房锻炼一下才来聚会。 “锻炼锻炼” & “锻炼一下”，they have the same meaning, so choose only one of them.

我晚上要现去健身房锻炼锻炼一下才来聚会 我晚上要先去健身房锻炼一下再来聚会 And, for the record. 再来聚会 means you are in the meeting room, or talking with the host. So "you will come back to the meeting". 再去聚会 means you are not in the meeting room, so "you will go to there soon". They both right, but with different meaning. So pick one on your own situation.