The name of the heirs (Crusader Kings 3)

The people mattered, the game for the tribal ruler, took over the kingdom, became the king of his first name, "Harald I", the son was born called him by his own name, he became Harald II, then there was Harald III, Harald IV, Harald V, and then again became Harald II, I xz as it happened, from his father always got a middle name, well, like Harald Harald Haraldson (once there was a third or a fourth I tried to understand why, but did not succeed). Tell me why this numbering went astray, it just became interesting, so beautiful looked, at least for me.
P/S. another question, it is already about another ruler, if one of the courtiers begins to blackmail that he knows that my current character is not the son of the previous character, and that my mother cheated with "such a brow" it means that it is 100% information that the current character is not the son of the previous?