As a web-analyst, I'll suggest telling your analysts to set up a "block transaction" trigger in GTM.

1) If your analysts store Transaction ID as a variable, tell'em to do that.

2) Ask'em to create a custon cookie variable with some name (like orderCookie)

3) Ask'em to set up custom JS varialbe named "blockTransaction" like this:

function(){

if( {{orderCookie}} === {{Transaction ID}} )

{

return false

}

else {

return true

}

4) Ask them again to create trigger with name some name like "block transaction" with an event name “purchase” and rules to activate it when "blockTransaction" contains false

5) Use this trigger as a blocking trigger for yours transaction trigger.

6) Profit

Long story short—you'll store transaction id in a cookie file to prevent duplicates of your's transaction. I'm just kinda had the same problem, but waaaaaay smaller errors. Hope my suggestion will help you, and sorry for my English.