Is it possible to be someone's uncle and grandpa?
Correspondingly, can someone be their own grandpa?
Suppose you marry a widow who already has a grown up daughter, and your father marries the widow's grown up daughter. Now the widow's daughter becomes your mother. Since your mother's mother is your wife, your wife is also your grandmother. As the husband of your grandmother, you become your own grandpa.
Similarly one may ask, can someone be your brother and your uncle?
For someone to be your brother, they must have the same set of parents as you. To be your uncle, they must have the same set of parents as one of your parents. There's no overlap there. That's still only a half-brother, since you and he have different fathers.
The only way to “become your own uncle” is to “become” the sibling of one of your parents. If you are not already the sibling of one of your parents, then you can't “become” the sibling of one of your parents.