Doubled dialog in export; not present in timeline playback

A strange, new-to-me audio artifact: Final export of straightforward master multicam sequence (includes dialog track, simple SFX, and music) via Media Encoder (as usual) kept resulting in files in which the dialogue was doubled – meaning it had a single echo, same amplitude as source. But none of the other audio clips were doubled, nor did it occur if I exported a brief section. It continued even after deleting renders and media cache. There are no submixes or other FX on clips or tracks. Finally, exporting directly from Premiere worked. So, seems like something in the ME pipeline? I obviously have a workaround this time, but need to solve this for the next project. This workflow and type of project is not new to me, but I suppose there are plenty of gotchas I wouldn’t know to look for. 

Thanks for any insights.