Hi there!
Ruben from the payments team at Monzo here.
We’ve recently been investigating a very similar issue, also affecting Tesco. We’re working together with some of our partners to reach a solution to this problem.
I presume what you might be seeing is that one of the two transactions is showing as “pending” (greyed out amount in your feed, and a comment saying “Pending Transaction” at the bottom of the transaction when you open it), while the other is settled (not greyed out amount, no comment of pending transaction).
If that is the case the pending transaction will release the funds automatically after a week, but if you’d like to have it released sooner you can get in touch with Support who are aware of the problem and will be able to release them sooner.
I apologise for the confusion caused here! 
Let me know if you have other questions and I’ll try to answer them 