Heb 9:11-14
"But when Christ appeared as a high priest of the good things that have come, then through the greater and more perfect tent (not made with hands, that is, not of this creation) he entered once for all into the holy places, not by means of the blood of goats and calves but by means of his own blood, thus securing an eternal redemption."
The contrast: earthly tabernacle vs. heavenly; animal blood vs. Christ’s blood; temporal redemption vs. eternal. Christ entered 'once' (ephapax) — the definitive sacrifice, never to be repeated. The redemption is eternal (aionion lytrosis).
Heb 9:27-28
"And just as it is appointed for man to die once, and after that comes judgment, so Christ, having been offered once to bear the sins of many, will appear a second time, not to deal with sin but to save those who are eagerly waiting for him."
The parallel human death / Christ’s death: both happen once. Christ’s death was to take away sins; the second coming will complete salvation. The certainty of judgment (krisis) is the context for the urgency of the Gospel.