In the following sentence, should the word because be repeated or is it better to express it just once?
In my opinion, a very good way to get fit is to go to the gym because you can work out whenever you want and (because) you can train all muscle groups.
The best and most idiomatic way to say this would be:
In my opinion, a very good way to get fit is to go to the gym because you can work out whenever you want and train all muscle groups.
But it would also be common to add "you" a second time like in your example:
In my opinion, a very good way to get fit is to go to the gym because you can work out whenever you want, and you can train all muscle groups.
Adding "because" a second time sounds weird, almost like it was written by a child, although it would be understood.
There is no need to repeat it. Once you have used the conjunction 'because' to introduce reasons for something you have just stated, everything that comes after it must either be part of those reasons, or else stand as a clause in their own right. The structure of your example leaves no ambiguity, it is clear there are two reasons.
It might be helpful to view your statement this way:
In my opinion, a very good way to get fit is to go to the gym because:
i) you can work out whenever you want, and
ii) you can train all muscle groups.
