inReach mini messages take three months to arrive....

The issues with AT&T were due to changes on the AT&T end (twice). AFAIK, they were resolved fairly promptly once the problems were reported. If you are still having problems, please open a support call.

I am unclear as to whether the messages sent in May were received twice (once in May and once just recently), or if they were not received at all in May. I have seen a few cases of duplicate message delivery, although not with anywhere near that long a delay between the original and the duplicate. Either way, you really ought to call tech support about this. There might well be something wrong with your account on the server end.

For what it is worth, sending to both emails and T-Mobile phones I've had no issues with delivery. 

Yea, this was an ATT - Garmin communication issue. I was hiking the AT when I figured out some of my family members (the ones on ATT)  were not receiving texts. To Verizon and others, it worked fine. I simply modified the contacts to include their email. Shortly afterwards, the issues was resolved.

Historically, ATT has had delivery/redelivery issues. I recall a time when my voicemails to my brother would show up three days later. So it may not have been Garmin's issue, at core. Because so many things are dependent on other systems now, it all has to be near-perfect uptime, for it to work as designed. The analogy, you're only as strong as your weakest link, comes to mind.

WD said:

Historically, ATT has had delivery/redelivery issues.

I've had AT&T text message issues from T-mobile so i'd say i agree with you that it is likely not a Garmin specific issue. There are times when i'll send a family member with AT&T a text message and it completely disappears into whatever places text messages go to die. 

This is mostly likely an issue with your providers SMS gateway and not with Garmin.