The Greek of Heb 10:12 reads:
αὐτὸς δὲ μίαν ὑπὲρ ἁμαρτιῶν προσενέγκας θυσίαν εἰς τὸ διηνεκές, ἐκάθισεν ἐν δεξιᾷ τοῦ θεοῦ
The KJV renders the text as:
But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God.
The term translated as "he had offered" is προσεωεγκας, the nominative masculine aorist active participle of the verb προσφερω, a sacrifical term meaning "to bring to/offer." It denotes how Christ's sacrifice (θυσια, the term it is coupled with in this verse) was done once-for-all in the past, and is not to be repeated.
The Vulgate renders the verse as:
Hic autem unam pro peccatis offerens hostiam, in sempiternum sedet in dextera De.
The Greek term προσεωεγκας is translated as "offerens" in the Latin Vulgate. As there are always difficulties and ambiguities due to translating texts into different languages, the Vulgate can be interpreted as speaking of Christ either as having had offered, or offering a sacrifice. However, the Douay-Rheims translated the Latin Vulgate as follows:
But this man offering one sacrifice for sins, for ever sitteth on the right hand of God
The Douay-Rheims grossly misunderstood the text of Heb 10:12 to present Jesus as presently offering a sacrifice, commensurate with the Roman Catholic teaching on the Mass being a representation of the same sacrifice Christ offered on the cross.
Of course, this is not the only time the Vulgate and/or the Douay-Rheims errs to promote Rome’s false theology (e.g., in Acts 2:38 where μετανοήσατε ["repent"] is translated as poenitentiam ["do penance"]).