A friend of mine is having a similar issue.
The low-fee transaction was indeed double-spent, but he did not get his BTC nor credited in his account.
He created a support ticket several times but got it closed immediately each time without the issue being resolved.
The only response he got besides a reference to some post of you about RBF was that "it will take a few days" for the transaction to complete.
There is no transaction to his address pending in the mempool, so it appears the issue in on your side.

Can you comment on this issue?