It is strange to assume a "scribal error" on the part of the Old Greek translators when the Leningrad Codex dates only to the 11th Century, whereas the Old Greek is a 1200 years younger witness. Yes if there was some complex hebrew or reference to an exotic animal or flower we can posit a translator error, but amen is a simple word.
There are multiple options
What many scholars believe is that the MT dates to a different textual stream, one dominant in Babylon, then the textual streams dominant in Palestine and Egypt. This is because with the discovery of the Dead Sea Scrolls in Palestine, it was found that they agree with the Old Greek much more than they do with the Leningrad Codex, and there was wide variation in spelling, phrasing and textual changes in different copies of the same books, supporting the notion of multiple textual streams circulating together. It is also believed that the Old Testament was not canonized yet and that there was disagreement as to which versions of texts were authoritative. There is evidence for proto-samaritan, proto-masoretic, and proto-septuagint texts in the DSS. Thus we now have three textual streams: Egypt (whose witness is Old Greek), Palestine (whose witness is the DSS), and Babylon (whose witness is MT), and in some cases they disagree with each other. In all cases, the proto-LXX hebrew texts have excellent agreement with the LXX and the proto-MT texts have strong agreement with the Leningrad codex. Thus there is no reason to believe "a scribal error" has to explain the difference as this could be different textual traditions. This is similar to the disparaties between Babylonian and Jerusalem Talmuds with rabbinical judaism prefering to follow the tradition stemming from Babylon rather than from Palestine.
Another option is that there is a single textual stream and during the intervening 1200 years some corruptions were added to the Leningrad Codex from the Hebrew copies made repeatedly over that period.
The translators of the Old Greek forgot to add the second amen during translation
Corruptions in our current copies of Old Greek
More options also exist. Readers can decide for themselves which is the more likely answer.
 EUGENE ULRICH. The Dead Sea Scrolls and Biblical Texts, published in Peter W. Flint, James C. VanderKam - The Dead Sea scrolls after fifty years_ a comprehensive assessment. Volume 1 (1998)