This seems like a simple error to me, the usual situation of your computer sending a payment twice.
However it does beggar the question of why the casino are not honouring your self imposed limits and rejecting any deposit over your limit – hardly in line with promoting responsible gambling.
I also wonder what would have happened if you had played through the whole lot and won £10k with your excess deposit – cue Redbet stating that it was not allowed because you had breached your limit 🙂 They do love a get out free alternative