Translation:My uncle gave me his helmet.
Probably the uncle who did not find his bike a few lessons back. :)
That's fine unless he piggyback rides on the motorbike and tempts the Three Fates.
does the 2nd "μου" need a tonus like "μού" in this case?
No, it does not -- it can't be confused with the possessive μου because if there are two, the first one has to be possessive and the second one has to be "dative" (indirect object), so there's no need to disambiguate with an accent.