"He was happy, because he won."
Translation:Ήταν χαρούμενος επειδή νίκησε.
Fully understood though, but it is usual in Greek to use the personal pronouns if they are necessary, to give emphasis to them. So double αυτός is not wrong, but if you use them, you should be sure that you want to give emphasis to the pronoun. A bit redundancy in this case. Double αυτός is giving too much emphasis to αυτός. You can even omit them, and the context shows who is the acting person. IT IS NOT LIKE IN ENGLISH, that personal pronouns are necessary, or French, but most likely it is similar with Italian, for instance, or Spanish. So, the rule of thumb is: Use personal pronouns when you want to show who is acting.
Please, Greek team, take a look at all these cases: They should be accepted, with or without pronouns, except for the cases they are absolutely necessary.
**The phrases that use personal pronouns when necessary are in good Greek.