Isaiah 52:6 (King James Bible) therefore my people shall know my name therefore they shall know in that day that I am he that doth speak
Here we have a clear case of dittography in the 1841 British edition. The typesetter had just set “yea in that day they shall know”; then searching his copytext for shall know in order to continue the typesetting, he accidentally skipped back to the preceding shall know, and thus he ended up twice setting “my name yea in that day they shall know”. The subsequent LDS edition (1849) removed this dittography.
Summary: Ignore the dittography in 3 Nephi 20:39 that the 1841 British edition introduced into the LDS textual tradition (but which did not persist).