mail merge: contents sent to email of next record

Build ID: 1:5.1.6~rc2-0ubuntu1~xenial2
CPU Threads: 3; OS Version: Linux 4.4; UI Render: default;
Locale: en-CA (en_CA.UTF-8); Calc: group

By way of example. I have 25 calc records used in the distribution of form letter to email addresses (one per record).
The first document gets emailed (twice) to the first two email addresses.
The following documents remain out of sync (document ‘n’ gets sent to email address of ‘n+1’)
The last document gets doubled up with the second last document.

The similar situation occurs when I ask the merged file to be saved as individual files, named ‘a’.
25 files are created call ‘a_0’ to ‘a_24’
files ‘a_0’ and ‘a_1’ contain the data from record one
file ‘a_24’ contains double data (driven by contents of records 24 and 25)

I have spent the whole day at this and am quite certain that it must be a software glitch.

Any ideas?

I’d suggest you to test with a later version, and if the problem persists, file a bug report at Bugzilla.