The “original” is like [name_f]Rivkah[/name_f], in Hebrew. רִבְקָה
Anyway, I consider [name_f]Rebecca[/name_f]/[name_f]Rebekah[/name_f] both okay. [name_f]Rebekah[/name_f] makes as much sense as a transliteration from the Hebrew as [name_f]Rebecca[/name_f], but is no more original/correct.
I think of [name_f]Rebecca[/name_f] as the more classic, historic English spelling. You’ll find it in more literature and it’s the more familiar spelling. For whatever reason while I prefer the -h on Biblical names like [name_f]Leah[/name_f], [name_f]Susannah[/name_f], and [name_f]Hannah[/name_f], I don’t think [name_f]Rebecca[/name_f] needs it.