Jeremiah exists in two lengths, and both are ancient

The ancient Greek translation of Jeremiah is about 2,700 words shorter than the Hebrew that English Bibles translate. It also arranges the book differently: the oracles against the nations sit in the middle of the Greek and at the end of the Hebrew. Whole passages, including the sixteen verses of Jeremiah 33:14-26, have no Greek counterpart at all. For centuries the natural explanation was that the Greek translator trimmed.

Then the Dead Sea Scrolls came out of Cave 4 at Qumran, and among them were Hebrew fragments of Jeremiah in both forms: some matching the long edition, and some, like the fragment scholars call 4QJer-b, matching the short one, in Hebrew. The Greek translator had not been cutting; he had faithfully rendered a shorter Hebrew edition that circulated alongside the longer one. Most scholars now read the short edition as the earlier of the two, with the long one a later expansion, though some argue the editions grew in parallel. Either way, two editions of Jeremiah sat side by side in ancient Jewish libraries, and nobody seems to have minded.

The best commentary on this comes from the book itself. Jeremiah 36 tells how King Jehoiakim sliced the prophet's first scroll apart column by column and fed it to his fire pot, and how Jeremiah dictated the whole of it again to his scribe Baruch, "and many similar words were added to them." A scroll destroyed, rewritten, and enlarged in the rewriting: the same book that comes down to us in two lengths shows, on its own pages, how that happens.