Subscribe by email sends up to 38 copies of the same email

We have used the Subscribe by Email plugin successfully for the past year. But this latest use literally "blew up" on us.

We have 179 users. When we are active, we have multiple posts daily.

On 23 June 2014 at 11:52 am only 24 subscribers were processed and successfully sent emails.

On 24 June 2014 2:12 pm 199 subscribers were processed 144 were sent, 55 are still pending. [144 is about the correct number of subscribers]

On 24 June 2014 10:20 pm 159 subscribers were processed, with 1 still pending. [Presuming 158 is correct]

On 24 June 2014 10:42 pm 281 subscribers were processed, with 122 still pending. [Again, 159 is probably correct]

And then, on 25 June 2014 7:44 am 2592 were sent, 2420 are still pending, and 172 is shown as the "Total". The problem is, there were actually over 3000 emails sent, and some subscribers state they received the email up to 15 times.

Needless to say, we shut things down.

After disabling the plug in, and removing the 2600 emails that were still in the pending queue, we enabled it again. It showed us some interesting things:

The first 21 subscribers (alphabetical by email) received one copy of the email. The next 11 received two copies. Then the patterns break up into 1-1-2, and 1-3-3, and progressively get worse. At the end, one user received 38 copies.

I do have the log files for those days. Started to post them here and thought better of it. Sorry for the dummy files attached, I couldn't find a way to remove them.