Why do Daily Office readings in the BCP 1928 sometimes repeat? Can anyone explain why the Daily Office readings in the 1928 (Episcopalian) Book of Common Prayer (containing the 1943/45 revision of the lectionary) sometimes repeat on Sundays readings that were read on weekdays during the previous week?

For example, the First Sunday after Easter calls for an Evening Prayer reading of Zephaniah 3:14-20 and John 20:19-31.  But Zephaniah 3:14-20 was already read on Friday Easter Week's Evening Prayer, and John 20:19-23 was read on Wednesday Easter Week's Evening Prayer.
I'm not a scholar of liturgical history, but the lectionary for Sunday's readings and weekday readings have followed separate cycles probably since 1561.  Additionally, both cycles lean towards giving readings appropriate to the season.  As a result things get repeated for the benefit of those who normally only show up on Sundays.

If you want to learn more, I recommend checking out haligweorc a blog by Derek Olson who really is a scholar of liturgical history among other things.