We had a perfectly civil thread going...
OP was mistaken as to her entitlement to TRS, therefore applied for it. Yes, that was a mistake but, according to OP, she honestly thought she was entitled. I also think it's not unreasonable to think that Revenue should know by their records who is claiming TRS and who is not and be able to cross-check to see that the same party is not claiming on two properties. Revenue appears not to have such a system in place, which I can also understand, considering the complexity of the various data they have to keep track of, but it is unfortunate.
It's always distressing when you make a mistake on your taxes and then get a bill much later than you expected - it's happened to me. I didn't blame Revenue, but that doesn't stop you feeling frustrated and upset about it. In fairness to OP, she's prepared to take the medicine and pay back the money and hasn't engaged in the wailing and gnashing of teeth that we've seen on other threads. And, thankfully, she didn't ask the question that we see a lot on other threads: "how would Revenue know that it wasn't my PPR?" So give her a bit of a break?
@Mommah, yes the interest relief on an investment property is potentially greater - 75% of interest payments with no cap, but it has to be offset against rental income (which OP does not have, as you say).