Is it obligatory to repeat repentance for a specific sin if a person realizes they were not ignorant of its prohibition at the time of committing it, after they had repented, believing they were ignorant?

If you repent to Allah with a sincere repentance that fulfills all conditions, then your repentance is sufficient, and you are not required to repeat it. Beware of obsessive whispers, for "the one who repents from sin is like one who has no sin."